Why is it that after working on something for hours and not solving it, as soon as you post for help, the answer is obvious? …
I post this as a “solution” to help others even though it is me admitting stupidity
The issue was my use of AND and OR conditions without using the brackets
I had
field1 = blank
AND
field2 = this
OR
field2 = that
I should have had
field1 = blank
AND
(
field2 = this
OR
field2 = that
)
which made it all work
arrgh 