Push Notifications Suddenly Using Tons of Cellular Data

Thanks for the reply. I neglected to mention this in my initial post, but I have already taken all the actions above that have been recommended. In addition, I also turned on Low Data Mode for Cellular (under Settings -> Cellular -> Cellular Data Options). Yet still Push Notifications continue to use cellular data. Overnight, while my phone was in my house the entire time, another 50 MB or so were used. This could be simply due to the amount of time it takes to update cellular data usage. In any event, I have been with T-Mobile for almost 4 years now and until now have never been surprised at the amount of cellular data usage and have never encountered Push Notifications using so much cellular data.

Update: I have contacted Apple Support. I had a call with a support technician and it was honestly quite tedious. It took almost 20 minutes just to get the individual to understand what was going on. One proposed solution was to do an enhanced logging session. I agreed to this, but the issue did not occur during the enhanced logging session. After this, I opted to just turn off Cellular data basically all the time. I have now tried erasing all contents and settings on my phone and did not restore from a backup. This also required erasing and re-pairing my Apple Watch. After going through all that I tried turning cellular data back on just to see what would happen. Within one day System Service again became the top consumer of cellular data, with Push Notifications and iCloud being the components under System Services using the vast majority of cellular data. I want to reiterate that I have turned off background app refresh on basically every app, turned off cellular data on most apps (including Photos and iCloud Drive), and have selected the "Low Data Mode" option under Cellular Data, and still the issue persists.

I am convinced that this is somehow a bug from the iOS 14.6 update on May 24, 2021, or maybe the watchOS update (7.5) that occurred at the same time. In three years as a T-Mobile customer I have not once been surprised by the amount of cellular data used, and the Push Notifications and iCloud components of System Services never even registered in the top 10 apps for cellular data usage. This is extremely frustrating because I basically have zero ability to control how my cellular data is consumed at this point.