'imports' shows all imports but 'photos' omits recent imports.

I import .jpgs etc from a Canon powershot camera and iPhone/iPad and all OK as they show up in the current Photos library in use on the iMac. The 'Imports' folder in Photos shows up to 15 Dec 2018 (last import) but images in the 'Photos' folder in Photos only show images up to 29 Sept 2018.

In the 'Photos' folder within the Photos app the first date showing is (1 Jan 1980) then 31 Dec 1979, when > is clicked it showing imports done a few months ago.

The iMac Photos library is not connected to iCloud ( only the iPad mini) so no conflict there I think.

Looking at the 'Pictures' folder in Finder I see 'Web Images.photoslibrary' showing the last modified corresponding to the last import date but only 5.8 MB in size.

There is also a 'Library.photoslibrary' folder showing 47.63 GB and last modified 13/12/2018 (not the last import date).

There are also iPhoto libraries and other '.photoslibrary's created from old iPhotos libraries in a folder in 'Pictures' but not showing any recent modified dates, and these go way back and are there for the record, they are separate re. size and subject, and to keep different users images in separate libraries.

There is also a 'Camera Uploads' folder last modified 11/12/2017. The libraries haven't been moved or tampered with in finder.

iMac (27-inch, Late 2013 - 3.5 GHz Intel Core i7) running OS 10.13.6 and Photos Version 3.0 (3291.13.210). Exactly the same problem on my partner's 2009 late 27" iMac - same OS etc. Both are backed up to external HDs.