Just because a word is listed as having associated kanji, that doesn't mean that the kanji are used on modern Japanese, or that Japanese people will recognize them as being associated with word.
I found 真逆 with the reading of まさか and the meaning that is listed here in the 大辞林 dictionary (a very reputable monolingual dictionary). You can see a reproduction of that entry on the website kotobank
This does not mean that you should ever use those kanji to write the word まさか, or that should ever read the kanji 真逆 as まさか if you see it. I would say it's far more likely to be intended to be まぎゃく if you see it in modern usage.
So, long story short, it's not a mistake, but it's also not common or known by most Japanese to be the way to write that word in kanji.
I don't know where you are seeing まっさか for this word though. I didn't find anything like that.
Understood, thanks for the reply!
まっさか on its own doesn't exist, but 真っ逆様 is a word with a suspiciously similar reading. Maybe that's where they got it from. It should be understood as a combination of 逆様 with 真 as a prefix, though.
Error in 真逆 entry?
Should 真逆 be まっさか - not まさか? I asked a native Japanese speaker, and he thought that 真逆 was not まさか. Also verified in another dictionary.