Slow motion not working

 I shot a clip in 50fps (pal) at a shutter speed of 50, and I placed on a 25fps timeline. I select speed/duration and move to 50%.  The video stutters, no smooth slow motion!  I view it frame by frame and can see that it only moves every second frame and not using all 50fps slowed on a 25fps timeline.  I've been doing this for years without a hiccup, and now since the update, I have this issue which I cannot resolve.  (Maybe it's a sign I need to switch to resolve!) To make it worse, it's random, some clips will work and some don't without any setting changes. I can no longer use slow motion on the clips I want! Yes, I've doubled checked the metadata, they are all 50fps at a shutter speed of 50 but some act as if they are 25fps.  Has anyone else experienced this ridiculous Premiere phenomena...... and solved it?

p.s I also tried (Modify > Interpret) with the same result. 

I'm using SONY a7IV 50fps clips, could it be the camera, the metadata clearly says 50fps?

I can send a sample camera video file if anyone wants to try it.