The drink holidays even pre-covid were constantly changing. I believe by 2019 it was down to only one per quarter, and usually it was lesser "holidays" like SWA's birthday or the anniversary of Chase RR Card. Even the image upthread is quite old, as it lists LIFT coffee which has been gone for years since the change to Community. No drink holidays have been communicated to flight attendants and none are listed on the current drink menu. You can infer what you want from those observations.
Also, I wouldn't read into when FAs do and do not charge for drinks. We get busy, it gets bumpy, we think we collected a coupon when we meant to come back for payment. You can try to theorize but I'll tell you it's all random chance.