From an old bug report (sent directly to devteam, June of 2017):
wand or scroll of create monster becomes discovered if it makes
a mimic that is concealed as an object or as furniture within
the hero's view. Fixing this in the general case [when does
seeing a mimic as something other than a monster mean that the
mimic is being seen?] is a massive can of worms, but fixing this
specific case is trivial.
PatR
03d7d64d15