This has not happened before, previously these files would be showing in the library but not be accessible. Presumably this is a change made in a recent update?
Unlikely, but possible that something in there broke. More likely that something changed in the way your NAS is mounted/unmounted when it shuts off (or the way Windows exposes it to client applications).
Turn off Fix Broken Links under Auto-Import settings. That will stop it from happening.
What is happening is that with Fix Broken Links set to Yes or Yes (Protect Network Files), MC will remove any missing files from the Library. With the special "protect network files" setting it tries to protect files that it thinks are on network shares that are missing, rather than actually deleted. However, there's only so much the logic can do for this (and it could probably use some tweaking). If the NAS volume exists, and Windows can browse the share, but it shows up empty, then MC will remove everything.
At some point when going down (or perhaps starting back up) the NAS share is showing up empty.
Now, on the upside, MC should "fix itself". When it removes files, it doesn't fully remove them. As long as the files come back in the same exact spot, if you just run Auto-Import manually, it'll go through and re-add them by "undeleting" them from the Library, rather than really re-importing them. That means that all metadata should be preserved.
But, better to not have it happen in the first place. If you're regularly turning off your NAS, it will be better to disable the Fix Broken Links setting, which will stop that feature entirely.