DOING THIS FIXED THIS ISSUE FOR ME...
I had this same issue too and I'm not sure if this will fix it for you or not, but it worked on my end. I had already offloaded the app and deleted the app and then I came here to see if anyone else was having this same issue. After reading through some threads I saw that people were still having issues even after erasing and resetting their iPads (so I'm not sure if this will work for those people), but I thought I'd add a step before I erased it all and it did the trick for me. I went to:
Settings > iCloud (in my Apple ID profile) > Manage Storage (right at the top under the bar that shows your storage usage) > Backups > iPad > and then I deleted that backup completely.
One note: When I saw there was an iPhone backup listed along with the iPad's I decided to check my iPhone's storage to see if Mail was using a ton of storage on my phone as well and it was only showing 2 gb instead of 20. Then I checked Mail's settings and saw that they were the same as what my iPad had so I wonder if somewhere along the line these iPad backups might be what is causing this issue? I chose not to delete the iPhone backup on my iPad after I checked this out.
Then I went to Settings > General > Transfer or Reset iPad > Erase all Content and Settings
Another note: When it reset it came to the Welcome screen on the iPad and I chose to use my iPhone to restore its settings. It's possible that you may want to skip that completely if your iPhone storage showed high gbs used, but since mine didn't I chose to try it. I wish I had written down a screen that came up later and asks if you want to do something, I think it was restore your apps maybe? I can't remember the exact wordage. I know that I chose the last option to not bring in anything or have them restore anything. Again I can't remember what the wording was. But I thought it's better to be safe than sorry in that instance. Then I just downloaded the apps I wanted and started from scratch.
It's worth noting that I don't use my iPad for much of anything besides tv/movie apps at this point so erasing everything and starting over wasn't much of a risk for me. If you're not one of those people I don't know what to tell you. But this worked for me. Just thought it might for someone else. Hope it helps!!