According to the internal WaniKani API, the kanji 表 has an auxiliary meaning “Front” of type “warn” with the message “Oops, we want the kanji meaning, not the vocabulary meaning”. Therefore, the intended behavior is that WaniKani should reject your input of “front” with a shake and the warning message, regardless of your user synonyms. However, a bug causes WaniKani to instead mark you wrong in case you have “front” added as a user synonym.
If you remove “front” from your user synonyms, it should return to the behavior of giving a warning instead of outright marking it wrong. Not a great solution, but at least better than failing the item.
More details about this bug are in this post:
@Mods, is this bug still being worked on? Or is there a problem replicating this issue? I just tested it in an extra study session and still encountered the problem.