This is a weird one I have yet to see answered online.
I have 200+ images I am editing and then exporting as a timelapse. In the Develop panel I develop the first image to my liking, highlight all, and press sync.
All of the images in my immediate field of view (about 12) receive the new edits (color, crop, etc.) but when I scroll to the next set of images, only THEN do they begin changing. I have left my computer on overnight to see if it just slowly works and....nope, I still have to manually scroll through all the images and wait as they process.
Well...I thought, maybe that's just the preview and if I export the images, they will actually be read to go! FALSE The images remain unaltered until I scroll over every single one!
-Up to date software (no update available)
It is just the preview and it's a known limitation. Lightroom only updates those previews that are 'in sight'. The only way to force Lightroom to update the others without scrolling to them is to select all the images and then choose 'Library - Previews - Build Standard-sized Previews'.