Riot alone has an order of over 30,000 miners, and they will receive the last batch by October 2021, this is one company out of the many large industrial-grade mining orders that hit Bitmain in late 2020, Bitmain staff and robots are probably working 24/7 but there is only so much they can do, the production of mining gears is capped by chips production from TSMC, large companies like Intel, AMD and Apple can hardly get enough chips for their production, Bitmain, and the other mining manufacturers are the "small fish in a big pond".
Also does anyone have heard something about when they will open their sales again?
Nobody knows, but we know it's very unlikely to be before September, I mean even if they re-stock next month, chances are you won't get the gears until the end of September, and that is being very optimistic, the Aug batch was sold in no time, I am pretty sure Bitmain only put a small portion of their products for the public as a "courtesy" to its small clients, but with whales like Riot joining the mining industry, small miners will be ignored and your will only option would be over-paying the resellers, which isn't something you want to do before thinking
twice 10 times.