Do you have WiFi Assist on by any chance, as if the phone detects a problem with WiFi, then it will change to Mobile Data.
Also you can see in the phones app settings which app is using mobile data.
Current Phone: Motorola Razr 5g I also have a Planet Computers Gemini
I work in IT as an Enterprise Messaging and Collaboration Tools Administrator (Office 365, Slack and much much more) and manage hardware both Mobile and Desktop / laptop, my desk is a bit like PC World
-- We are all customers and dont have access to any o2 accounts