Goto the App Store, click your user icon in the upper right, click purchase and click "Not on this iPhone". The list will show the install date of apps listed in the order deleted (top being the most recent removal). There is always a chance you forgot about one you deleted, we are human after all.
If it's not that app offloading is typically the issue.
App offloading is the automatic process of removing unused app and reinstalling as needed to conserve storage space. If you goto Settings > General > iPhone Storage > Offload Unused Apps and see its enabled then that is source of increasing uninstalled app cellular usage. Without YOU uninstalling an app nor it appearing to be uninstalled you'll get Uninstalled Apps usage due to iOS offloading the app.
"Uninstalled Apps" itself is just the database of the static sum total for apps that aren't installed (these apps do not have a network usage state so they can't access cellular data). "Uninstalled Apps" isn't an app nor does it have cellular network access. Meaning if App X has 100 mb usage and then is uninstalled it was show 100 mb usage additional to "Uninstalled App". App X can't add more data without it first going through the list again as an app with a "true" network usage state (allows app to access cellular data).
Although I can't speak for every bug in iOS it is safe to assume an error causing Uninstalled Apps to increase is just from mismanagement of the cellular list labeling scheme. Meaning the overall usage is accurate but its reported incorrectly due to app thinning, file downloads of associated files for a deleted app (ex. mkv file with VLC being the player but deleted), cache data downloading, etc. not an app that is actually uninstalled secretly using data. This would be an app functioning at a high level to use data without the core framework to actually function as an app at all.
There could be some sort of remote notification bug that persist on an app that was removed but I doubt that could really be a thing for a couple reason nor do I think it would add up to a significant amount. But anything is possible I guess. OR maybe old version of an app that is "updated" with significant changes requiring re-submittal and review is included in "Uninstalled Apps" since the update is technically a new app. Don't really know just tossing ideas out there.
For those with the time and geeky enough to bother I would recommend downloading Xcode and using the provided tools to watch iPhone data usage.