Seems like a bug then, can you report it at tracker.ardour.org? Sad to say you will have to sign up for a different account as it isn’t tied to here at this time but otherwise it may get lost on the forums here. Thanks!
Just one more thing: I have tested in Ubuntu Studio 23.04, with Ardor 7.5, and also in Ubuntu Studio 24.04 beta with Ardor 8.2, and it happens exactly the same.
Maybe it’s not a bug and this is the expected behavior?
The issue is rather specific, if the playhead is between two bars the action “Playhead to Pref/Next Grid” locates to the prev/next bar. … and then gets stuck there.