Welcome and thanks for taking the time to join our community. I'm sorry to hear about the difficulties with your mobile web allowance.
It can depend on whether your phone has lost its Wi-Fi connection at any point. If mobile data is active on the handset then it will use the mobile web allowance if the Wi-Fi connection drops.
This can happen even if no apps are in use as mobile data can also be used in the background as well.
Any questions please do get in touch and let me know.
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