This should be reproducible using version 1.5. To reproduce do:

echo -ne "TITLE=title1\nARTIST=artist1" > tag.file
metaflac --import-tags-from=tag.file file.flac`
metaflac --list file.flac

You'll notice that the file.flac has the TITLE tag, but not the ARTIST. The tag is silently ignored, which may not be caught until a much later date. I'd say it should be considered on par with the severity of a dataloss bug. So at least issue a warning if this won't be fixed in the near future.