That was Henry Cavill being called back for reshoots after principal photography had already wrapped months before. By that point he was already working on another role, and the mustache was part of it. He made a perfectly understandable decision to say “I’ll do reshoots and help you fix this dumpster fire you made, but I’m still committed to this other role, so fix the stache in post”.

Yeah kinda but the guy could have still just shaved it