I have not used the Web UI of Calibre before, but I suspect it's going to be a Calibre bug.
There was a similar bug in Calibre ( see
https://bugs.launchpad.net/calibre/+bug/1945889 ) that caused this exact issue (metadata missing) with books added through the auto-add folder feature of Calibre which got fixed in 5.29.
I'll try to get the Web UI set up and do some tests, then I'll report that bug to the Calibre dev to hopefully get it fixed.
EDIT: Can confirm, that's a Calibre bug, will report to the developer.
EDIT 2: Here's the bug report:
https://bugs.launchpad.net/calibre/+bug/1992244