XnView Software

Checked, fixed in v1.5.2. Thank you.

Installed v1.4.5 this afternoon. (Not Portable, full install)
Downloaded XnViewMP-win-x64.zip, unzipped and launched from unzipped directory. Bug not here !
View2.jpg

So started a fresh VM, unzippd in the VM and launched from unzipped directory. Bug still here !
View.jpg

So, seems it's indeed ...

The preview panel is of bad quality on XnViewMP v1.5.0 x64. Looks as if image was expended from a low resolution copy.
No such issue on XnView Classic, XnViewMP v1.45, or other software such as FastStone.

Those 2 screen shots were taken Out of the box with portable versions:
Preview XnViewMP.png ...