Mobile data not working with unconnected WiFi switched on

[stoofa]

OK, I need to 100% confirm this situation, but thought I'd post here to see if anyone has any ideas.
Wife as a Galaxy S6 on Vodafone. Latest software, etc.
When connected to WiFi everything is working fine. She can browse the internet, use social media apps etc.
If however she is away from a WiFi network, unless she switches WiFi off she cannot use the mobile data.
So for example she can be in the middle of town and she might have a 3g signal and the WiFi icon is indicating it is not connected. Mobile data will not be working. However as soon as she switches WiFi off, mobile data starts to work.

Has anyone seen anything like this before? Any pointers?

Cheers.

 

[stoofa]

I forgot to reply to this one. Smart Network Switch is set to off. Wife confirms she had this issue yesterday.
She was sat on a bus. WiFi was showing as not connected, She had an "H" signal strength, however she could not do anything over the mobile network. Switched WiFi off and then she could.
I have an S6, also on Vodafone at the same software level and as far as I know I've never seen the same issue.

I shall attempt to be in a situation where she has the issue and I can immediately try and replicate it on my phone.