Uninstalr lists portable programs to be uninstalled. Typically I just delete them, but since this particular program was listed (Blender 3D software), out out curiosity I tried to use Uninstalr. To my surprise, it listed many completely unrelated programs in the Registry Data to be removed (some of them - in theory - I no longer have in my computer). Below is a partial list. Needless to say, I just deleted the Blender folder manually.

HKEY_CLASSES_ROOT\Appusermodelid\c:/portable programs/handbrake/handbrake.exe\
HKEY_CLASSES_ROOT\Local settings\Software\Microsoft\Windows\Shell\Muicache\ : C:\Portable programs\Microdicom\Mdicom.exe.applicationcompany
HKEY_CLASSES_ROOT\Local settings\Software\Microsoft\Windows\Shell\Muicache\ : C:\Portable programs\Microdicom\Mdicom.exe.friendlyappname
HKEY_CLASSES_ROOT\Microdicom.application\Defaulticon\ : @
HKEY_CLASSES_ROOT\Microdicom.application\Shell\Open\Command\ : @
HKEY_CURRENT_USER\Software\Microsoft\Windows nt\Currentversion\Appcompatflags\Compatibility assistant\Store\ : C:\Portable programs\Blender-2.90.1-windows64\Blender.exe
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Compatibility Assistant\Store\ : C:\Portable Programs\MediaCreationTool2004.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows nt\Currentversion\Appcompatflags\Compatibility assistant\Store\ : C:\Portable programs\Microdicom\Mdicom.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows nt\Currentversion\Appcompatflags\Compatibility assistant\Store\ : C:\Portable programs\Mpc-be.1.8.2.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows nt\Currentversion\Appcompatflags\Compatibility assistant\Store\ : C:\Portable programs\Naps2\Naps2.portable.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appbadgeupdated\ : C:\Portable programs\Mpc-be.1.6.1.6845.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appbadgeupdated\ : C:\Portable programs\Nomacs-3.14.2\Bin\Nomacs.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Blender-2.82a-windows64\Blender.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Blender-2.90.1-windows64\Blender.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Doublecommader\Doublecmd.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Dummy file creator\Dummy.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Fiji.app\Imagej-win64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Handbrake\Handbrake.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Hfsexplorer\Bin\Hfsexplorer.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Losslesscut\Losslesscut.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Losslesscut-win-x64\Losslesscut.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.5.5.5274.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.5.5.5433.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.6.5.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.6.6.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.6.8.5.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.7.2.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.7.3.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-be.1.8.0.x64\Mpc-be64.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Mpc-beportable\App\Mpc-be\Mpc-be.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Naps2\App\Naps2.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Newsflash plus\Newsflash.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Niouzefire.1.8.0\Niouzefire+.exe
HKEY_CURRENT_USER\Software\Microsoft\Windows\Currentversion\Explorer\Featureusage\Appswitched\ : C:\Portable programs\Nomacs-3.8.0\Nomacs.exe

What version of Uninstalr are you using, and was the portable version of Blender located at C:\Portable programs\Blender-2.90.1-windows64\Blender.exe?

The location for Blender is correct. I'm using Uninstalr Portable 2.6.0.414.

Thank you for reporting! The issue should be fixed in the version 2.7 that is scheduled for release tomorrow.

I say "should be fixed", because we have tested this issue and it doesn't reproduce on our side so we cannot confirm whether the fix works. It would be great if you could try the version 2.7 tomorrow and confirm whether the issue still exists.