i had the same problem: not being able to search the comments field.

but i found a solution, for me anyway:

just press the "Spy Glass/Search"-icon.
you'll get a pop up named 'Search'
and you can fill in 'Comments' whatever you search for in comments

Thanks for the links, yes I would much prefer a fix. If no-one responds with one, I will email them. Thanks.

I do not have an answer to your problem....
But if you would like to go Retro Monkey...here is where to look for earlier versions.
I, personally, would hope for a better solution than downgrading!

Edit: That link is for official releases only. If you want to get a Beta that worked for you, go here and find your last version that worked for you.

If no one knows, how can I revert back to the prior installation of MM? I don't want anything to affect the database.
Or better still, can the search function be added back?

Using the lastest release of MM.
I collect live shows & in the comments tag of my FLAC library I include the name of the taper / person mastering etc.
Prior to updating to the lastest MM last week, if I entered the name of a taper in the main search box, all that tapers shows would be shown. Now they are not.

That information was only in the comments tag of the FLAC file, not in the album title etc or anywhere else, so it must have searched the comments tag.

I cannot see how to turn this back on?

I ran the latest install over the older MM which was the previous release (non beta) version.