iPhone 7 Plus here with iOS 11.0.3.
This is what is happening to me; When I wake up the iPhone from sleep (wifi is connected), the screen first shows LTE (Cellular), the it immediately shows WIFI Symbol again.
This happens usually. I wonder, when the phone is in sleep mode and the WIFI is dropped, if the LTE is activated automatically. If so, I am eating up from my Cellular network's Data, and that ain't so good!
However, when I observe the Cellular Data Usage from iPhone's Menu, it seem no Cellular Data is used, the last number is still there. (Let's say the Cellular was at 230 MB, before it went to sleep mode, and when I woke the phone up in the morning it still says 230 MB.)
Seem like only a screen/display glitch. When the iPhone goes in to sleep mode, the display turns to LTE and when I wake the phone up, the diplay shows LTE, but immediately goes back to WIFI Symbol again...
As far as Wifi drops, if I am far away from my Modem, WIFI turns into LTE for a moment, but when I get closer to the modem, WIFI symbol comes back on.
I seem to remember WIFI range was better with iOS10. I might be wrong, may be it is that display glitch that shows LTE instead of WIFI. What I am trying to say is I used to see WIFI symbol with iOS 10, when I was far away from the modem. But with the iOS 11.0.3, same distance from Modem, iPhone (or at least the display) switches to the LTE.