I also experienced this bug, however i am on windows and it only started today after the new client update.
Basically it shows your Pokemon card (whoever is in first slot in my experience) each time i end a battle by fainting a poke or even when i run from a poke, I also tested it with trainers and it also happens after each fainted poke in trainer battle (have not tested in pvp will probably try that later)
Im not sure if i can post videos here or i would because it is fairly easy to recreate.