Hi @marco82 ,
I am not sure if I clearly understand your implementation.
However, I believe you should be able to link a the review to proposal – post object –
You will need to pass the the post ID to the review form. from the review form, you can then hook into the acf/relationship/query to load that specific post object.
https://www.advancedcustomfields.com/resources/acf-fields-relationship-query/
Hopefully this helps.