Not my experience.It’ll always use Wi-Fi if Wi-Fi is active and connects to the interne. On iOS 14 and probably 15, you’ll get a warning message pop up telling you or asking you that it wants to switch to cellular if the Wi-Fi internet goes down.
I just got a pop up message two days ago asking me if I wanted to switch to cellular data when my Wi-Fi went down. iOS 14.8 …. But maybe that’s because I had switched cellular data off for that app. That’s probably why.
I have Wi-Fi assist turned offI’ve never seen this message.
From time to time I have been switching my wife router off, or changed it with another, and I’ve never seen such a message.
Did you switch on a specific setting for it?
EDIT: unless it is hardware specific? I have an SE (2020).