iphone – How is app using cellular data after selectively turned off?

One of the apps set to Disallow use of cellular data (Solitaire) uses the most cellular data of any app. This is despite the switch Always being set to Disallow use of cellular data.

Is there another way to block a specific app from using cellular data?
I don’t want to turn off cellular data altogether because my low budget provider doesn’t hold mms for redelivery(!).

I have turned it on (moved switch to Allow use of cellular data), restarted phone, turned it off (moved switch to Disallow use), and restarted phone again. And restarted many times since. It still uses cellular data.

(extra clarification notes)
iPhone 7, ios 14.4.2, WiFi Assist is always disabled.
On the Cellular page of Settings under the section heading Cellular Data is the list of apps that have attempted use of cellular data. For the user to allow or disallow use of cellular data on a per-app basis is a switch next to each app. I have set many apps to Disallow use of cellular data, including Solitaire, which continues to use cellular data anyway.

It’s not alone, others like Backgammon and Paper.io also used cellular data despite being switched off. But Solitaire uses the most data, hence the focus on Solitaire.

switch off but still top data user