I've seen similar issues crop up over the years. The most likely root cause is that the auto import is running when the NAS is temporarily not available (whether due to the NAS sleeping, needing to spin up drives, or just some networking quirks). So some number of files are appearing to be "no longer there." Autoimport, when fix broken links is set to "yes," will remove any "missing" files from the database. But on the next import, because the files were only ever temporarily unavailable, autoimport reimports them, which sets a new date imported.
To rule this out, check your autoimport settings. If you have "Fix broken links" set to yes, change the setting to "no," and see if the problem goes away. Of note, the "protect files on missing drives" setting for fix broken links in my experience mostly works well, but is not perfect. Set the setting to "No" for testing purposes and see if your issue goes away. If it does go away, you can either try the protect missing drives option, leave fix broken links set to no and handle any issues with broken links by hand, or you can work on improving your NAS availability, etc.
As for how to fix the issue for files already affected, you can try rolling back to an older library database backup (JRiver makes them automatically), but apart from that, I'm afraid you'll need to manually change the dates back.