There is really only one reason that MC will change the import date on already imported files: This happens when the original file "disappears" and the "reappears" with a new name. You are using a NAS which is a HUGE clue to this!
Many NAS devices will not be ready all the time. Many of them go to sleep to save power. As a result, many times they will not be ready to serve files when you first ask for them. This can make it seem like files are "gone" to MC for a short time, while the NAS is still waking up.
Unfortunately, MC ships with two options set that cause "reimporting" on files.
1. Tools > Options > Library & Folders > configure auto import > tasks > fix broken links
This is set to "yes" from the factory, which causes unavailable files to be deleted by MC. I set mine to "NO" as this is the safest and will NEVER remove my files unless I tell MC to do so. The other selection "Yes protect files on missing drives" might also work for your situation. I do not use it. Try setting this to NO and this particular behavior can't happen again.
2. Tools > Options > Library & Folders > run auto import in background
If this option is checked, any time MC sees new files in the auto import directories, it will import them. Convenient! It works well in most circumstances. But in this case, where files disappear because they are unavailable... and then re-appear seconds later, this makes MC think the files are brand new! So it re-imports them. Turn this option off and run imports manually whenever you need to. If you find this too inconvenient, just turn it back on. It can lead to unusual behavior every now and then, but it's not nearly as invasive as #1 above.
There's another possibility though:
If you, or a program you use, renames files or directories on your NAS (where the music files are), then this same thing will happen: MC will see the old files "disappear" (because you renamed them) and then reappear with new names. Again, leading to auto import finding them as new, importing them, and giving them fresh import dates.
Hopefully this helps you understand , diagnose, and fix your issue.
Good luck.
Brian.