If I remember correctly, there is only ever one generation of backup stored in iCloud. Especially in scenarios like yours with different software versions, this is not enough. My recommendation is both still keeping the previous iOS/iPadOS version as long as Apple still supports it during the transition period. And additionally to iCloud backups, do manual backups to a PC or Mac with iTunes/Finder. There you can have multiple backup generations.

I ran into this a bit over a week ago when I upgraded. I had been running the public beta on my 2020 iPad Pro, and upgraded 2 days after 18 officially launched, but after it was pulled for bricking M4 Pros. It sounds like 18.0.1 is in progress and should be out soon, however I installed the 18.1 public beta on mine after seeing others have success with it from 17.7.

That said, my prior upgrade sat around for at least a week before I threw the beta in and there were no issues.

If you have another iOS device, you should be able to go to settings>Account Settings (the one with your Memoji)>iCloud Backup and see all backups stored in your iCloud.