I have a problem with copying and pasting in Notes.app.
If I select and copy text from notes in the main Notes window, I often see random snatches of text from elsewhere in the same note instead of the text I have selected. This is deeply annoying, particularly as I use Notes to store things like code examples, URLs etc. It's also highly irritating when trying to move bits of text around inside the same note.
However, there is a workaround. If I double-click on a note to open it in its own window, this problem seems to vanish, and copying and pasting works just fine.
Obviously it is great to have this workaround but it is very difficult to always remember to open a note in its own window. It is more work than I should have to do, and it took me some time to figure out and verify.
These problems have even led me to consider replacing Notes.app entirely, but there doesn't seem to be another free notes app that does everything Notes.app can, let alone one that provides a notes import process.
The copy/paste problems with Notes.app reported here sound exactly like what I am experiencing.
I am running Notes.app version 4.8.1 (1872) running on MacOS "Big Sur" 11.5.2 on a 2019 13" MacBook Pro.