This has been a perennial problem with my own generated comics with both Nano Banana Pro and ChatGPT (all generations). I often have to put in an extra edit to erase the false signature. It is annoying and I'm unsurprised most users don't bother.
For certain values of "my own".
Given that they can reliably do this, I'd think it should be trivial for the harness to automatically insert a "review the image for anything that looks like an artist's signature or other blatant indicator of plagiarism, and fix it" pass.
There's a program for that on your computer, it's called 'true'.
And then people wonder why the default mood of AI is so pessimistic. It's just revealing all of society's broken windows and adding a few more in the process.