Strange iCloud backup issues with Messages

[JohnSmithy]

Hi there. So, this is rather strange, and I'll try to describe it best I can.

TL;DR: It doesn't seem as if my Messages are part of my iCloud backup. They don't show up as part of the storage bar chart. I also "magically" gained back tons of iCloud storage after enabling and disabling Messages in iCloud?

Here's how things went down ~2 weeks ago:

• I've had 50gb of iCloud storage for a long time. I was sitting at around 42gb used out of 50gb for almost a year.
• I woke up one morning -- after no significant changes to my phone whatsoever -- to an email stating I only had ~100mb of iCloud storage remaining, and I should consider upgrading my storage. That's weird, I thought...
• Looking at iCloud Storage on my phone, it did in fact show I was right at or just over 50gb. Again, this is strange since I hadn't done anything or made any changes, etc.
• One odd thing was that the backup size of the iPhone itself displayed at 40gb or so, so why was it saying I was almost at the 50gb limit? Anyway...
• I keep all my Messages backed up on my phone. It's like 17gb or so. I do not keep Messages turned on under "Apps using iCloud" since I don't need it.
• Here's the weird part: I enabled Messages under Apps using iCloud, and after realizing that I in fact did not want that turned on, I turned it back off roughly 15sec later.
• After doing this, my iCloud storage suddenly dropped to 32gb of 50gb used?!
• I have absolutely no idea what happened. However, I want to be sure my Messages are backed up. I understand that Messages are included in the standard iCloud backup, which differs from the "Apps using iCloud" portion (I always found this a little confusing, but I digress).
• My iCloud storage chart is almost entirely a purple bar of "Backups", and the remainder is a sliver of "Docs" and "Mail". Nothing else, e.g. no Messages at all.
• If I look at the detailed breakdown of the backup itself (Settings > iCloud > Manage Storage > Backups > iPhone), Messages is not on the list, but e.g. Photo Library is there.
• This all happened as of about 2 weeks ago, maybe a little more. I thought I'd give iCloud backup time to "settle" or fix itself, but I'm still sitting at 32gb of space used.
• Just a side note that I have verified that my iCloud backups have been successfully backing up each night, i.e. it's not failing to backup or anything like that.

Another oddity -- if I look under the local storage for the phone, under Settings > General > iPhone Storage, I can see that Messages is taking up 17gb. Photos is 32gb, which corresponds to the amount of photos being backed up. Furthermore, if I drill down into Messages and select Top Conversations, I noticed they're all listed at "-1 byte"

I suppose that my concern is whether or not my Messages are in fact being backed up to the cloud. If Messages take up 17gb locally, shouldn't that be included as part of the backup? Why would it be missing?

Thanks in advance for any advice or suggestions!