In all likelihood the person that fought you switched out all his dragons just before attacking you (switching dragons in arena tab doesn’t get registered. Suppose you normally have the following dragons on your team: Light fury, Toothless, Hookfang, Copsekeep and Gyllentalen and just before you start a fight in arena you switch out Hookfang for Tricky 2 heads. If you then go check the log afterwards you’ll see that the system thinks you brought Light fury, Toothless, Hookfang, Copsekeep and Gyllentalon with you in the fight instead of Light fury, Toothless, Tricky 2 heads, Copsekeep and Gyllentalon. and it will consider the hookfang as being defeated even if all your dragons stayed alive. )
It is a bug that has been around for a long time. I reported it a while back when i seemingly got defeated by a person that didn’t bring any dragons with him at all.