Thanks ... I got to understand that while I was writing the post, but left what I wrote as is, to illustrate what I think is legitimate confusion ending up with data corruption.

The UI for that new facility is bad. There is nothing to indicate that it is an update facility in the menu item name, nor in the dbox heading. ... It is a "lookup" facility, which does an update without requiring confirmation, and with no undo facility.

And it was not me that set the checkbox on, the one that is (hiding) at the bottom of a long list ... It's was defaulted to On.

Its dangerous IMO

It's to find an alternative match for the Album you're showing. So selecting an alternative match re-tagged the files, thus they show with the new match you made (if you have update tracks enabled in that Context Menu).

Right click on the files and use Find More from Same > just like in MediaMonkey 4.

steps:
[list]I am in the detail view for Album A Love Supreme by John Coltrane[/list]
[list]I used the "[url=https://www.dropbox.com/s/h97iqnru8t22qea/lookup%201.png?dl=0]Lookup Alternate Album[/url]" feature, and viewed the album Coltrane by John Coltrane ... my expectation was that this is MM5 version of MM4's Find More Of Same facility[/list]
[list]the response is confused me because [url=https://www.dropbox.com/s/co99fsrdfby86yh/lookup%202.png?dl=0]it showed[/url] the release date and wikipedia entry for that other album, while still showing me the cover art and track list for the original album[/list]
[list]I tried a few other albums to try and understand what was happening, and be able to describe "the buG" to you[/list]
[list=][b]==>[/b] [i][url=https://www.dropbox.com/s/jn9q2we7oyyuim3/lookup%203.png?dl=0]database corruption[/url][/i]: If I view all of my John Coltrane's albums, the original album has disappeared, and its tracks have been merged into the 2nd album ??[/list]

comments:
[list]if this is intentional it is really bad ... I had no expectation that I was doing an update, the menu function said "lookup" ... I do now see that there is an "update tracks" checkbox at the bottom of the lookup dbox ... it is bad enough that it is in a neutral colour at the bottom of a long list, and I didn't expect nor notice it ... but why the f@#k is it defaulted to on?[/list]
[list]I opened a "lookup" facility, and selected an item from a list ... and you updated my tracks without confirmation? [/list]
[list]if the purpose of the facility is to update (vs lookup), the menu option should be something like "[i][b]correct[/b] album details[/i]"[/list]

I now have mess to clean up caused by the use of your "lockup" button ... ie. "A Hard Day's Night" by The Beatles is now inconveniently filed as "A Hard Day's Light" by The Foo Fighters, etc