Some extensions, as Variables, do some actions in wikitext that affect more than than the given wikitext snippet. This can lead to unexpected behavior if code paths are expanded even though they aren't used, as happening in the #varexists function. This is inconsistent considering the behavior of ParserFunctions and #var default values.
It's really easy to fix this, I actually did it on the fly to improve the efficiency of the function. However, this is a very little breaking change, so I'll increment the version number.