It's a shame that there don't seem to be any primary sources from Christians of that time explaining that practice. I would imagine that the Desert Fathers would have mentioned it at some point (if it happened in their time), but perhaps they weren't aware.

Since this practice doesn't seem to be a part of Christianity elsewhere, it seems like it could something unique to a coptic interpretation of scripture. I'm not qualified to interpret this - but it does appear that a coptic translation of leviticus 21:18 uses a word which is translated to "cut off nose" ( https://coptic-dictionary.org/entry.cgi?tla=C5694 ) in for a word that is usually interpreted as "flat nose" or "deformed nose" elsewhere.

Whether there's some other connection between that word and idol worship, who knows.