Not sure if this was already reported or nah since search didn't yield any results about it and I do not remember all the reports from testing.
Basically bug is that pokemon can't held z crystal if they don't have a responding move to the crystal. Not really game breaking or anything but it can cause problems for some mons in competitive and I know that reborn is trying to be as close to main games as mechanics go.
In gen7 any pokemon can hold ANY z crystal but you get a message if you won't be able to use it. So yeah a chimchar can hold a Pikashunium Z if you feel like it