With update alpha2-g3bad925 I've some disconnections on 5 GHz band that instead I never had with previous alpha2.
There's been absolutely zero changes related to wifi between the two alpha builds. Your problem is your environment, not the new firmware build.
People need to stop always thinking that new firmware versions are magically responsible for every issue (or fixes) related to wifi. There's barely any change whatsoever related to the wifi code between new firmware versions. The wireless driver hasn't changed in over a year. Just because a problem appears or disappears after a firmware upgrade does NOT mean that this is related to the new firmware version. Often, it's simply because of the reboot itself that things changed. If you had wifi set to Auto, then your router just happened to move to a different channel.
A few years ago I did that little experiment, where I distributed three firmware versions, asking people which one gave them the best performance. What I didn't tell them was that two of these had exactly the same wireless code. Turns out some people still reported seeing a difference between the two... "Signal is stronger with this one".
This is why I mostly stopped caring about wifi-based feedback years ago. Too often people fail to do even basic troubleshooting, instantly blaming a new firmware version for any change in behaviour.
In short, start by troubleshooting your whole environment, instead of systematically attributing the source of any wifi behaviour change to a firmware version. Wifi performance is function of the router, the clients, and the local sources of interference.
I use that same firmware code at home myself. The last time I saw a real, genuine issue caused by the router's wifi driver was back to the RT-AC87U days, where the Quantenna driver had a well-known issue with some Android devices. Since then, I have never seen any difference at all with all the different firmware versions I've ran on my RT-AC88U.