I was able to find a workaround by quickly typing a single letter when the On-screen keyboard popped up, and quickly pressing B (On Xbox). This was after several attempts at creating signs where the text would disappear. After typing "Y", that sign and the future Signs/Name Tags ended up working. It may or may not be reproducible - I will comment further with findings.
Seems like really the only reliable work around is luck and brute force. Just keep spamming signs with a single letter until one of them sticks. You'll have a window of time to place more until you get the privilege taken away XD.
Seems to be resolved on my Realm, however - do not want to celebrate too early as it could just be a side effect of the Realm server being restarted for the in place upgrade to 1.19. It could also still affect other users, regardless it's good to hope that it is resolved after 8 months of waiting.