This bug seems to affect a lot of stuff besides pet battles; many quests (some hidden ones) have been reset account-wide after a character transfer. There’s a thread in the Bug Report forum with more information.
https://us.forums.blizzard.com/en/wow/t/pet-battlecollection-broken/681040
Also, for what it’s worth, my pets were knocked down from 2,000 to 1,000, not from 1,500.