Just did it again. I have been testing, opening from files both with and without saved tabs. I had rebuilt the database, and it seemed to be behaving for the last week, under all conditions. Now I just downloaded a batch of photos from my android phone to a folder, and opened by clicking on one of them. Upon entering folder browse view, all thumbs in that folder are mis-associated, including the file just opened. No tabs were saved on last close before this. Upon reflection I believe there may be a correlation with opening from newly added files, which perhaps have no system thumbs generated yet. That may be a full correlation actually, there doesn't seem to really be a correlation to saved tabs. The substituted thumbs are all from the same folder as each other, another folder on the same drive, but in a different area. I have not accessed that folder at all recently using xnview or nautilus. Thumbs for all other folders are not affected, just the folder with the file I opened from. File names are correct, preview images at bottom center of browser are correct, all thumbs in browser gallery for that folder are wrong.
I just tried opening from a file from a different folder, one with no recently added files, where all files are known to have good thumbs in both system and in xnview. There were no errors, browser gallery displays correctly. Even in this case with opening to a second instance of xnview. I would say the correlation with errors in thumb associations when opening from recently added files with no system thumbs is correct. I believe this may correlate with all occurrences of the issue. I hope this helps.
Again, I'm using XnView MP Linux Version 0.86 64bits (Apr 25 2017) on Ubuntu 16.04 LTS, 16gb ram.