Makes sense. Mylar bags are designed to be used with oxygen absorbers mostly. The humidity from the moist tobacco in the tins escaped and created humid conditions in the mylar bags that was conducive to rust. Throwing in an oxygen absorber would have prevented the issue.
If humidity escaped from the tin in the mylar bag, that same humidity would be escaping and drying out the tobacco if left unbagged. The seal on European style tins is not totally airtight.