Time Ramp not rendering in-between frames

Usually, Premiere Pro will take a moment and then render a 50% slowed down clip, where I usually opt for Optical Flow or Frame Blending, however neither work. I simply get repeated frames without any blending. Tried clearing media cache.

I have tried to RENDER EFFECTS IN WORK AREA however it just plays the unrendered version, with a yellow line in the timeline. This worked for 20 years until now, please assist.

Win11, latest PPro. High spec machine.

UPDATE: I have not found a solution - sorry