No shame on you (only), until now I also assumed this option was still a little 'broken'.
After checking sound signature in the same way as the OP did, MB gives the impression it just failed, giving no additional information at all.
It would at least be an improvement if MB would give some feedback after checking the sound signature.
It has 'fooled' me all this time...
What seems really erronous is that when 'update blank fields only' is ticked, also really empty tag fields are not filled.
I just checked with 'track6'.
Album name, publisher etc. are also not filled when 'update blank fields only' is ticked, even though they really are empty.