I’ve been able to workout a solution to this and I’ll post it here for anyone else who might find it useful.
Basically I asked myself what if… the originaldate tag didn’t exits in my files, would plex fallback to just use the date tag? It does!
So, I removed originaldate and for good measure originalyear so my tags updated to this …

… and then done force refreshed the album (didn’t even need a plex dance) and boom the full date came in !

So I think what I’ll do to workaround this is a two step approach:
When I initially add my tags I’ll make sure to set date = originaldate, something like this$set(date,$if2(%originaldate%,%date%))but will need to test some more to get the code right.Then do a second tag update to JUST removeoriginaldateandoriginalyearleaving everything else untouched.
EDIT: Was able to do the above with one step by adding the following code to my musicbrainz scripting under options:
$set(date,$if2(%originaldate%,%date%))
$delete(originaldate)
$delete(originalyear)
I believe this will give me fairly accurate release date information, at least it should be better and no worse than I have now.