When saving an album some metadata is not set, and it's not possible to enter it manually in windows either.

The issue seems to be Windows isn't reading the tags if they aren't near the beginning of the FLAC file? I'm not entirely sure what this means but there's more information in the link below.

There's a thread here which figured out a workaround for this issue, even though it's really a Windows issue:
https://hydrogenaud.io/index.php?topic=118153.msg975223#msg975223

Would it be possible to get such a work around for Picard too? (Basically write the meta tags before writing the album art)

I've attached an image showing the issue.