I've been using mobile data while I thought I was on a wifi signal. I've downloaded an app to check if this is what is going on and it looks like it is.
Mobile data is being used while I'm on my Vodafone wifi.
I do not have wifi assist on so that can't be causing it.
Also, I asked the Vodafone online chat help person and they have told me that every time I enter a wifi area unless I manually turn off my mobile data on my phone, I will not connect to the wifi but will instead be using mobile data. News to me and also my phone bill as up to this month as I have NEVER had this issue before joining Vodafone wifi. Apparently, all technicians will advise me of this and it is standard practice to turn off mobile data in a wifi area. A bunch of hogwash or am I losing the plot?
As your probably aware Wifi assist will actively let the phone choose the best signal connection available where you are. Like you I turned this Off on the iPhones I've used and my s7 edge.
If you leave the Mobile Data setting turned On in your phones settings then if the wifi connection dips / drops then the phone will search out a stronger connection which can be the Network data signal.
The best way to control what your phone can connect to is to turn Off the Mobile Data Setting in your phone which will only let it use Wifi.
All the phones I've owned have performed in a way that if your in a Wifi area and wifi is turned on in your phones settings then it'll try to connect to it unless you have tweaked the wifi permissions settings to not join new networks which some phones have the settings to allow that tweak.
The amount of times I've been surfing the net on a bus and using 4G / 3 G etc and my phones switched to Wifi which has been slower than the network connection is unreal which prompted me to tweak my wifi connection permissions.