This is deeper. Because guild was "community"-ized, it was affected by an RP addon block:
https://www.reddit.com/r/wow/comment..._get_it_fixed/
https://us.forums.blizzard.com/en/wo...roleplay/45663
TL;DR an RP addon was using communities to chat cross-faction, causing Blizzard to lock down communities from addons, which explains the use of the |Kv#|k obfuscation.
Also, I missed the mention of "Ambiguate" in OP.
That function has been around for a few years and has nothing to do with message content. It simply formats a character's name in an attempt to standardize "name" and "name-realm" display across the UI. It is not defined in public code. I've tried using it before, it's not very reliable.