iphone – iOS Messages phone storage usage different from iCloud

I have a conversation log in my iPhone (iPhone 11 Pro, iOS 14.0.1) that goes back to 2009. iOS says it is occupying 15.96GB:

ios screenshot showing 15.96GB phone storage usage

I have the phone set to save everything in my conversations (e.g.: high resolution images and videos) and I don’t want to change this setting.

I want to back it up in iCloud so I upgraded to 200GB and connected my phone to WiFi and a charger. It started displaying the “Uploading messages” message and progress bar at the bottom of the screen. This message and progress bar no longer shows up and it has only uploaded 8.1GB:

ios screenshot showing 8.1GB iCloud storage usage

This number doesn’t change anymore regardless of me charging the phone or not. I don’t have Messages iCloud sync activated in any other device. Any idea why this seems to have gotten stuck?