How is table_1::field_B defined? It seems like it would have to be a number to evaluate properly. Typically, Boolean is zero and non-zero, as opposed to empty and not empty. Does that guide you anywhere?
"Pure" Boolean is 1 or 0. However, FileMaker allows for 0, 1 or "" (empty). While I have been known to use that "feature", if you validate the field to always have the 0 or 1, (set data to 0 by default?), then you will never have an empty.
0 ≠ "" (empty)
true (1), but obviously FileMaker lacks a Boolean variable type. So, in FileMaker, zero is false, non-zero is true. To deal with empty number fields, I use (myNumber > 0) to provide a Boolean 1 or 0.
Thank you Beverly and David.
The field type of the condition input was text, the behavior of this conditional formatting is unpredictable unless using the function isEmpty(). I tested with a number field, then the behavior was predictable: empty or 0 = false without using any function. So the return status of the formula window should display "undefined" when a raw text field ( "table_1::field_A" ) is used as input for conditional formatting.
Could it be that booleans are internally represented by numbers, and that a function is required to make a type casting if the field is not already a number?
To expand on Bevs response..
You can also test using built in constants true and false where true = 1 and false = 0