multi relation field supposed to contain array of values, record IDs. even if it’s single ID, or no ID
it’s still array
“contains” suppose String 1 contains String 2, not array.
You can create additional formula field near multi relation field with CONCATENATE(‘multi relation field Name’). It will contain the same, but in a form of a string, and formula will work.
But I feel that your goal can be done even in more simple way, i just can’t realize your goal.