That’s most likely the same issue as Are there technical problems at AcoustID? - #18 by outsidecontext, where AcoustId randomly returns no results. I already reported that to luks
There can be the odd case where Picard gets matches from AcoustId, but the metadata matches so badly Picard would ignore them. In this case lowering the Options > Advanced > Matching can help.
If you can provide the debug log we could see what happened in your case, but my bet is on the AcoustID issue with random empty results.
No, because if AcoustID gives you no result there is no AcoustID bro save.
The generate fingerprints action just generates the acoustic fingerprints locally, but does not query AcoustID. That’s mostly useful for submitting fingerprints.