I am having this issue where I can't move a layer on top or under another layer.
Layers can be locked or unlocked. It won't place it. It goes two layer up or two layers down.
Sometimes it works other times not.
gosh, I though you were talking about FCP sorry, no wonder I was confused.
It works here though. Moving groups and assets up and down in Motion can be tricky at times but I can definitely move them anywhere I want.
Have you tried to create a new project and copy and paste it in there. Sometimes I do that when I encounter weird behaviour with motions and seems to fix the issue. Other than that, maybe you can try 'reset preferences'. (Press and hold the Option and Command keys while opening the Motion app)