In your case, it is refusing the save the section when you use this fragment in a label with a tree grid. It would be interesting to see if Tracer reveals anymore information about this.
I just tried this with a section hosting a treegrid in 7.2.2 and I don't see the error. Is it possible for you to put this scenario into a simple sample under test ruleset that I can import at my end and verify? That would be faster. Thanks.