Will put a metallic plate behind an RFID antenna change the readings?
Most known RFID methods are digital, and incorporate some means of self-consistency checking.

Therefore, the actual identity "readings" would not be altered.  What would be altered (likely substantially reduced) would be the chance that any given reading attempt is successful.

If you have a reader that reports some kind of signal quality metric, that could be altered.

Ultimately your question is a bit too broad, as going beyond the simplistic question, the meaningful question requires speculation about the nature of the unspecified device.