now that does ring some foggy bell. Gave mine away a year ago including the braincells with the usage knowledge.
But wasn‘t it in the former cases that 2nd WiFi would not startup at all? Summary at that time was somehow: when I remember correctly, LuCi incorrectly reused the same Mac/BSSID over, when creating additional WiFis in LuCi. To fix, you had to manually edit the wireless file afterwards, to remove the duplicate value on one of the WiFis.
there a several old threads, this is just some of them