• That’s normal for multiple choice

    No it isn’t.

    sometimes all the answers are correct

    Not in multiple choice they aren’t. At best you might have “D) All of the above” if there’s genuinely more than 1 answer

    We can assume

    Someone screwed up somewhere, and there will be lots of complaints from students. Despite it being “you only had 1 job”, proofreaders still miss things sometimes…