If my clipboard has exactly 4000 characters in it, it'll paste. If it has 1 more character, the paste is empty (clears program)
Edit: I can copy more than 4k characters from the factory manager, it's only paste that doesn't work

I'm using the 1.21.1 version of the mod. I haven't used other versions, so I don't know if it's specific to this one, but it looks like the only relevant difference is the lack of the program size limit in sendProgram, and that was slightly larger than 4,000 (32,600. Payload limit?)