Add Freeze Frame not working

There was a previous thread on this at https://discussions.apple.com/thread/253051148, started in 2021, but it

I'd never had the problem before, but just in the last couple of weeks, working with my usual camera footage on a fairly up to date system (FCPX 10.6.5 on Ventura 13.3.1 (a) on MBP M2 Max) with no identifiable recent changes, I suddenly can't create freeze frames, exactly as described.

The workaround at the end of that thread (to turn the offending clip into a compound clip and then freeze that instead) works just fine, but I'd love to know what's going on and have a proper fix back to how it's always worked before.

When I try to do the usual freeze frame (either with keyboard short cut of from the menu 'Add Freeze Frame') it just shortens the clip to the playhead instead. I've checked that my edit settings hadn't changed to some tiny length of still, but it's set to my normal 3 second choice. (I also tried changing this to 2.5 seconds to see if I could nudge it into life.

When the clip shortens, zooming in I can see it has created a one frame clip - but extending the out point just opens up the moving image file - not a still - so what the command seems to be doing now is splitting the clip and creating a one framelength clip at the split point.

Apart from the workaround in previous thread, it doesn't seem like there were any actual fixes.