So I checked all my Priority Pass logins to see which ones show BRU:

US Amex Bonvoy: yes
US Citi Prestige: no
US Chase Sapphire Reserve: yes
EU Amex Plat: yes
EU Visa Infinite: no

The only pattern I can see here is that cards that DO NOT include restaurants in general have access to Diamond lounge at BRU, and vice-a-versa. but that's probably a coincidence (?)