You code snippet gave a hint:
Hint: Local property "Prop" is not used
So I assumed this feature was called "Local property".
Anyway, yes, it seems to be some new feature in (obj)FPC and Delphi doesn't support it.
JCF also has more issues with newer features in FPC.
Even de code formatter in Delphi sometimes has hiccups with newer features but at least that one doesn't check syntax. I found the requirement of correct syntax really annoying in JCF Formatter in Lazarus.
You could try to post it as a bug in the bugtracker but I'm not sure the JCF Formatter is actively maintained for new features.