From a bug report: a shapechanger
which becomes a mimic would always stay as 'm' rather than take on object
or furniture shape. Same applied to monsters which hide in other ways.
The code did that deliberately, but I don't think that it's actually
necessary so this lets them mimic/hide when they're in the right shape.
If they change form to non-mimic/hider while concealed, concealment ends.
It would be fun to have shapechangers-as-mimics actually change their
mimicked shapes periodically, but this doesn't do that. They'd probably
change to non-mimic quicker than they'd mimic something else so it's not
worth the effort.
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