Bug/Issue Add/subtract groove event not working properly?

So i was messing around with making custom groove effects, and i noticed something really off abut the add/remove unit groove action.

When setting groove to a specific value, it works fine, but when adding or removing a specific amount of groove it... Doean't do the thing it's supposed to do. The amount of groove changed seems to be completely random.

I spent some time testing it, but i couldn't find a strict pattern for the behaviour. I would run the same event multiple times in a row, and sometimes the unit would gain a lot of groove, sometimes a small amount. Sometimes, when i tried to subtract groove, the unit would end up with more groove charge than she started with!

Has anyone else had the same issues?
I'm on the switch, in case that matters.