Your silence on this is deafening.
I know this is now almost a year later, so not sure if you have more info now, but I suspect that Alpaca were pressured to block those stocks by the market makers.
As I understand it, one of Alpaca’s main sources of revenue is Payment For Order Flow (PFOF), where the market maker (e.g. Citadel) pays the broker (e.g. Alpaca) a small commission per traded share to route the order through them, rather than directly through the exchange.
Some market makers really didn’t want retail investors buying AMC, GME etc. so they probably threatened Alpaca with some sort of termination of services if they didn’t comply. While controversial, PFOF allows Alpaca to offer its customers free trading, so blocking those stocks would have been a tough decision for them.