Ah I see. Your tag references "merged commit" but not "merge
commit", former being implicit part of the latter when it comes
to "git describe".
Printing HEAD for 2.066 shows this:
commit 2b795569cd892801050faec80054d91c1fb54d3c
Merge: d84aadd 601d478
As you may notice it mentions d84aadd (your v2.066-b3 tag commit)
as on of merged parents.
May I ask what command sequence have you used to create the tag?
Doing this locally result in proper HEAD tag for me:
git fetch upstream
git checkout 2.066
git tag testtag HEAD
