Maybe not mouse then if keyboard function is also affected.
I just did a bit of a websearch on "right click context menu missing ubuntu 20.04".
One result mentioned Theme may be the issue and suggested trying default Theme.
Also, do you have another DE, other than Cinnamon, that you could boot to do a context menu test.
Just seen your other post: Zorin Back-Up Info Needed - #32 by Frog
If you are now back on Gnome, is the problem still there or gone?