Custom ASCII character mappings
Ignoring customAsciiCharMappings for the moment, Craft uses Stringy for this type of thing.

The have some special logic for German and Bulgarian character replacements and it looks like those particular characters will only get replaced if the locale of the site you're currently editing is bg: https://github.com/danielstjules/Stringy/blob/master/src/Stringy.php#L1874-L1877