Just raising this again, as it seemed to be fixed last year in a firmware update, but has since regressed.(refer https://community.tp-link.com/en/home/forum/topic/670654?replyId=1393670).

Issue:

I have three X20 Deco access points.

When my phone is connected to the same x20 device as my chromecast dongle - I can not cast to it.
If I switch my phone to another x20 access point, I can cast successfully. This is repeatable every single time.