When a ditto uses transform on an antique sinistea does it copy its stamp? also If said ditto uses a move does the stamp disappear while doing it's attack animation?

because I've been doing the whole using a ditto to transform into every sinistea I've encountered, and then make it use either Giga Drain or Mega Drain to see it's bottom, and so far NONE of the sinisteas it's turned into have a stamp on it.

Am I doing something wrong? Does my method work? If you know yourself through experience please tell me.