In my case after some tweaking I noticed what was causing this behaviour was changing the default beacon interval to 300 (from 100), I restored it to 100 and the problem hasn't happened again, so......

-Confirmed, last night, even though some devices stayed connected, speed tests through wifi gave a mere 5Mbps download 1Mbps upload, after rebooting the AP500, speeds increased to the maximun I can...

I saw this too, we are on 2017, is this device abandoned by tp-link?

I have a similar issue with 5Ghz (haven't tested if the issue happens with 2.4 yet). Devices which still connect may take longer to connect (like an ipad 2), and another device gets disconnected more...