1:1 previews behaving strangely

I have a folder on LrC with apprx. 1800 photos. I was noticing that browsing the images in "zoomed mode" - e.g. 50% in the Library module was slow. It kind of needed to load ("Loading...") each image before it became sharp - kind of the same behaviour as when there are no previews available - or maybe just standard previews. Normally there is no delay in browsing as long as I have generated previews. I have a Mac Studio M2, which is very fast in LrC.

I used the "Discard 1:1 previews" menu item, and tried to generate previews once more. The really strange things is that it used "normal time" to generate the first 600 photos, and then suddenly it just completed the progress bare in "no time" - it told me that it either had created or there existed preview for 1800 images.

Some images are now fast to load and some are slow to load (as if there are no previews). It seems that there really is no previews for all images.

Any ideas? I have tried to optimize catalogue - but not help. Is there a limit to how much preview space one can use? I have apprx. 468GB og previews on my local disk. I guess one solution is to delete the previews folder, but it is not good to loose all previews already generated...