Some of my photos do not show up in Photos after editing however they are OK after transfering to an external drive as .jpegs

I have been using Photos for some time and it has always worked well. But lately some of my photos do not display in Photos, i.e they hold a place in the library but as a blank screen. However, I can make them appear by engaging the edit function but they disappear again after closing edit. I can export them to an external drive as .JPEGs and they are fine, complete with edit changes. So, I am not actually losing these photos, they simply do not display in Photos. I have upgraded to a Nikon D850, so the files are definitely larger than earlier, e.g 50 to 60 MB, but there does not appear to be a clear correlation between file size and whether or not they display. Any ideas?