I have "clear existing tags" set in preferences, and the following script:
$copymerge(composer,writer)
$copymerge(lyricist,writer)

You can see the issue I ran into in the screenshots: somehow, rather than being merged, the lyricist credits were duplicated. But on digging deeper, it turned out there was both a flat string and then the separate multi value tags.

If I manually delete the string value and save the file, then then when I reload the release, everything looks right. So, not sure exactly what's going on. Maybe it had been previously saved when tags were being flattened, but that still should have been cleared when it was loaded again, right?