Language.UNSUPPORTED_FORMAT_PATTERN is intended to catch stray %d and %f placeholders in the translation resources and convert them to %s, since the translation component only supports this.
However, the regex is currently too broad – in particular it will also catch and convert %%dark and %%food (to %%sark and %%sood respectively), despite these being escaped.
This does not directly affect players, but it's a landmine waiting for mod developers and also your own dev team.
Affects resource packs in vanilla as well.
Simply fixed by checking if it's escaped or not, or by simply removing the converting since it has been deprecated for a ton of versions and vanilla is entirely converted.