On my wife’s phone (which has not received any updates since I created that poster), my contact card shows my avatar image only (which I have shared via Messages at some time in the past), not a monogram.
After quite a bit of web searching, it appears that the only way to “go back” is to delete and re-create your contact. The recommendation is to export it from a Mac and re-import after deletion. The data exported does not include the poster.
This procedure is supposed to work for posters you receive from others, but it doesn’t work with your own contact card (that your phone will send out).
I found several Reddit discussions complaining about this problem - that a poster can’t be deleted, once created. Apple’s got a bug they need to fix. I sent a report via Feedback.