I’d have to see all the code of your filter.
But, if you have two different fields with 2 different field keys then this is the issue. The code example you’re following is designed to work with fields that have the same field key. In other words, the same field/field_group assigned to both post types.
Sorry if I missed anything where you said it was two different fields.
I have a different example that does this for two different fields with different keys here https://github.com/Hube2/acf-filters-and-functions/blob/master/acf-reciprocal-relationship.php
