For the same period of time — since about the date of the first post in this thread — I’ve had a problem that looks the same but is actually a paste problem, not a copy problem.
After some action, the last-copied text isn’t pasted using ⌘v. I know that the text was copied because I see it in my clipboard manager, Pastebot. However, ⌘v does NOT paste the text that was last copied; it pastes the text that was copied immediately prior. The result looks like the copy didn’t work but it’s actually the paste that didn’t work.
Once I paste from Pastebot (instead of using the keyboard shortcut, ⌘v,) then this problem goes away and ⌘v pastes the last-copied text as expected.
I hadn’t reported this because I hadn’t done the legwork to determine what causes ⌘v to access the wrong item on my clipboard stack. I had assumed that it was related to some mistake in my date-formatting PopClip Extension.
I know that this paste problem is not related to upgrading to macOS 15 Sequoia; I’m stuck on the last version of macOS 12 Monterey because my iMac won’t support anything later. Another weird thing about my setup is that I have a second clipboard manager, CopyPaste. But it’s not running while I’m having my paste problem so this shouldn’t make any difference, right?
Anyway, perhaps the OP’s problem is with paste, not copy.