Which device are you on?
I experience the same thing (or white instead of black, sometimes also clear).
I’m on Android, using several devices - issue seems to be on all of them.
All assets are downloaded.
Here are some screenshots, all from the same battle.
The empty/transparent one (Thrall) happened after I got hit, the rest no idea.
They sometimes change color, too, as you can see.
I’m getting the same white-outs. I’m on Android.
Thanks for the info!