Which dragons are you fighting? There are those that have abilities that bring back previously defeated dragons at a lower health. The Chestnut Knight and Knock-Rocket do this, off the top of my head. I don’t think any others do?
Of course, if this is happening just randomly and they’re not brought back by the use of another dragon’s ability, it would be a bug. You could contact support in-game about this, but if you would rather go the route of email, to contact them would be email@example.com. Whichever way you decide to go, if this is indeed a bug, be sure to include your support key with an explanation of events.
Also, please be aware that support is still under an influx of tickets, and it might take a while before anyone is able to get back to you. Hope it gets figured out!