I have found the same issue with other apps too, but this is the one I remember. So, I am not sure if it's an OS level issue or an app issue.
I using an app named "4k Video Downloader" (it downloads youtube videos). When I edit the name of the file before saving it, I am unable to paste text with ⌘V but I have the option to paste in the context menu. I have also observed that the app does not have an edit menu on the menu bar. Am I unable to paste via the keyboard as the app does not have edit in the menu bar or is it some other issue?
This is an observation that I have made and as a new Mac user, I just trying the OS better. This is not a support request for "4k Video Downloader". I have observed this in other apps too, but I decided to dig into it after I observed in this app.