BNBs in the Vault are not (entirely?) included in the 30-day BNB average

Despite what is written in the FAQs, which turns out to be false, BNBs in the Vault somehow do not count (maybe not entirely but this doesn't change anything) for the 30-day BNB average. I'm 100% sure of that 'cause I experienced it firsthand: despite I was holding more than 1 BNB in the Vault for more than 30 days (which correctly kept my BNB average above 1 and my card level to 2 for some time), one day the BNB average started dropping for no reason (I never touched my >1 BNB in the Vault), it got below 1 and my card level fell back to 1 (and I lost the 2% cashback on multiple transactions too). As I moved my BNBs back to the spot wallet the BNB average slowly got back above 1 and the card level got back to 2. No answer from customer support, they just dropped the conversation. Twice.