It appears that keywords are required to be in all caps in filtering expressions. This is either not shown correctly in the Help, or else it's broken in Mp3tag (2.46a).
One other thing that bothers me is the loose syntax, where either '%fieldname%' or 'field' both work in a filtering expression. It should be one or the other, not both.
'fieldname' is faster and can search in all parts of a multi value tag
Ah, ok. Now that I look at it again, I see that I was reading it wrong. I was looking at the 'plain language' explanation of the examples. The problem with the Help page is that the plain language explanations are in some cases identical to the examples.
artist IS Air AND album IS Moon Safari
Artist is "Air" and album is "Moon Safari"
The plain language should be something like:
Find files where the artist field is "air" and the album field is "moon safari".