I had fooled around in the meantime, because I have also observed a delay in picking up tag and files changes for my files which were residing on a Synology NAS.
For this I moved my files to a local disc and after that my WORK and TAG problem vanished for the cases reported. I’m not sure about the reason, because I did a “force re-scan” several time on my NAS before and expected this to pick up any changes… Will look for similar occurences.
What I have noticed however, is the system behaviour for one track “works”. If the PART tag is left blank (which would normally be the logical thing to do), the work won’t show up in Roon unless copied to the TITLE tag.
I guess I could live with that.