Wrong. Until OE complete, they don’t even need an excuse. ALPA would still try to help you, but there ain’t squat they can do until OE completion. Even after OE you don’t have full protection, but there must be cause.

Having said that, the number of pilots fired in their first year is exceptionally low. Like, really low. And it’s not for stuff like this. The key here is “prudent planning” (like, return early if there was a winter storm, not taking the red eye back, no backup, etc. ). If there was a simple unforeseeable mx delay/divert, and you got back late, you’ll still have a job.