There are several threads about the 4 point edit Fit to Fill option being greyed out. I too have experienced that. Solutions offered (of which none worked) centered around "resetting"/cleaning out caches etc.
Within the last month the television station I work for issued me a brand new laptop. Knowing it was an issue I moved nothing across from the old one and built my workflow from scratch. The problem still persists.
I have noticed it randomly will appear as an option. But I really couldn't find a reason or thread why. It wasn't until version 24.5 started crashing a lot and one thread emerged of Premiere not handling MXF media correctly as a possible reason of the crash that the light bulb went off in my head.
I think the common thread I have in Fit to Fill not working is when MXF media is my source. Which for me is about 90% of the time as I shoot news on a Sony XDCAM series camera and edit that material. When I had mp4 file video from our archive system it seemed to allow the Fit to Fill function. So there could be several things within the MXF ecosystem that have been "broken" in some recent updates.