XnView Software

Select a folder with a lot of images, Xnview MP will start making thumbnails.
Scroll with the scrollbar to the bottom of the folder and click on an image.
If you now scroll up with the scrollbar (or the mouse wheel) you will notice that the thumbnail creation stops (the progressbar on the bottom of the window stays at the same percentage).

XnviewMP 0.98.3 x64

are you able to reproduce each time?

Yes, I can reproduce it every time.

Deleted the whole XnviewMP folder and did a new installation from the zip file.
Started XnviewMP without changing any setting and I got the same result.

on Windows 10, i can't reproduce

generating previews is a sore topic of this program, even after transferring files, sudden folders or their changes, previews are not updated, the program does not follow the content update at all, because of this I had to put the content update and preview button on the panel
On the subject: the generation stops even if there are about 20-30 images in the folder, this is some kind of strange bug.