I know topic is old, but this is the first result in Google looking for the solution, so I will post it here. I installed XnViewMP Version 0.83 x64 (Oct 3 2016).
I also had problem with thumbnails not refreshing in windows folders after editing them. I could crop them, rotate, change colours and windows thumbnails stayed the same. After editing pictures with other programs, windows refreshed thumbnails correctly. XnView browser also refreshed them correctly. What others propose are more workarounds rather than solutions.
The problem is in XnViewMP option of saving edited image without changing it's modification date/time.
In Tools -> Options -> General -> File Operations I unchecked options for "keeping original date/time".
This solved the issue.
Thumbnails won't refresh
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