I just noticed this issue on my iPhone XR. I got a notification from my carrier that I’ve used 90% of my 8GB of data and I have 11 days left in my billing cycle. My data usage is usually around 4GB per month, so this was pretty strange. I noticed that System Services and Uninstalled apps we’re taking up the majority of my data usage. I Reset my statistics and am keeping a close eye on this now.
This seems to have started around the time I installed iOS 13.3.