My phone does this.
My work place has wifi I’ll often walk away or to the bathroom and Have to turn off wifi on m phone during the walk because it struggles to load, as it desperately tries to cling to the wifi.
I have wifi assist turned on and still doesn’t help. They need maybe a time out. Once 5-10 seconds has passed use LTE.
+1.
Whenever I fly on a WiFi equipped plane, I have to turn the WiFi off on my iPhone 6 or the LTE doesn't pick up when I turn off airplane mode. This also happens before leaving the gate if they turn on the WiFi early. The phone connects to he airplane's WiFi and then waits for the log in/authentication and it gums up the LTE.
FWIW
DLM