Looks like it's specifically when an underscore follows a normal letter instead of whitespace. Not sure if that's as intended by the markdown spec, incorrect in Voyager or unspecifed

That's almost certainly intended behavior, otherwise any underscore would trigger formatting (much like the asterisk does instead of allowing snake_case_naming

Underscores do not work inside a word (so my_long_identifier won't accidentally become mylongidentifier, but _foo_ will become foo)

That may be a lemmy specific way to markdown

Ah, gotcha.