Roon is importing only half of the files scanned on drive

Did you actually check that or do just believe that it is so.

And pointing the all new Roon to the same old share as before didn’t work the same?

It’s hard to figure that out when changing all the parameters/variables (new Roon, new share, …?) at the same time.

Also in the new working setup or just in the old, now no longer existing one that exhibited the issue you reported?

All in all, this seems to be just a continuation of your earlier thread (or at least you provided the same log snippets as being relevant):

I don’t say that Roon is error free software, but scanning files (as a base component of the software) usually works robust. You reporting partial scans only sounds suspicious to me. As Roon just tries to read in the files, there isn’t much Roon can do wrong here I believe. To me this all sounds like one has to search the root cause on the NAS. We don’t have all the details nor access to the NAS to check our self.

Searching the forum, I found only one other thread reporting the same issue (sadly never got resolved). This user believed it might be related to btrfs as filesystem in use.

I fear troubleshooting your NAS (OS) is beyond what this forum category is meant for, and with that little details about it and a now error free working setup, I don’t see how to help further. Just enjoy your now working setup and report back if you run into issues again in the future.
Maybe @noris has something to share from the analysis done on your older thread?