It's not a glitch, it's absolutely normal. The Good Deeds meter typically goes up every time you successfully praise it, like normal, and it goes down when an evolution occurs. I like to think of it as your character being in a new stage of life and so now it has to "rebuild" its good deeds. Not from scratch though, typically from the halfway point if you evolved into a healthy character. It doesn't change anymore after you become an adult, if I remember correctly, unless you are feeding it snacks and the bat successfully steals your character's treat away. This feature of the Good Deeds meter fluctuating isn't exclusive to the Angels, it also happens for P1s and P2s as well (if not all vintage Tamagotchis, come to think of it, because most of them have discipline meters).