(Screenshots on my phone right now, will add later.)

When I enter the end, the ender dragon was alive for a while (makes sound, health bar visible). When I started to destroy the ender crystals, it somehow died itself - did NOT make death animations or sounds, health bar disappeared, exit portal activated, egg spawned, its model appeared somewhere else (in its stuck-in-minecart animation, player cannot interact with it). I'm not sure if it happened before or after I destroy the ender crystals. I CANNOT reproduce the problem after creating a new world.

Info at the top of the screen:
Beta 1.2.20.1 (cli-p260, s2094833656) Gui:3.00
HUAWEI VTR-AL00, Mali-G71, Android 7.0

Additional info:
Phone: Huawei P10
Game config: default mode is survival, I was in creative. Chemistry turned on. Difficulty was peaceful. Gamerule domobspawning turned off.