I was doing some battle quests earlier with my T2 pets and after finishing lvl 10 I moved on to challenging normal opponents (Erdoo). The two times I fought them it told me I'd completed the Battle Quest once I defeated them (giving me the 450 exp, item + moving me to the next level of BQ). I figured at first it might have just been super coincidental and I somehow chose the same challenger the BQ told me to fight (since I hadn't bothered looking once finishing lvl 10) but after I realised I double checked that I was choosing to fight someone else (BQ was asking for Nependeathicus and I was fighting Manticat) and it still told me that I'd completed the quest.

I had the same thing happen to me twice already as well. I finished after level 21 cause well I cant defeat Saggitarius and quit both those and got the gate keeper and I cant beat him either so i went to battle a cash shop challenger The Haunted Woods and got you completed the Battle Quest and got the exp for it

I'm having the same issue and it's very confusing

It is happening the same with me!

I want battle but I am winning all the times the quest o.o

I've been experiencing the same issue. I can't really go on to beating opponents outside BQ, because anything I beat counts as completion of the quest. This glitch isn't particularly game breaking, but is a mild inconvenience for us battlers, as we can't fight without accruing extra, unearned exp, and thus, inadvertently cheating.

This continues to be an issue. It could be especially frustrating with the upcoming Fireside event challengers...