I've been using 1Password for years and never have I had this issue. But I have one that started last week sometime.
I sync my 1Password vault between my iMac, my iPhone (always my CURRENT one) and my iPad with Dropbox. I only have those 3 devices, I have checked on the Dropbox site and have verified this.
I can see my iMac sync and see the updated file (new modified date/time) itself in Dropbox. I also copy files up & down from Dropbox with other apps & scripts. No issues there.
I've created new test logins on my iMac and they do not get synced with either my iPhone or iPad. I've created new test logins on my iPhone AND iPad and they do not sync to my iMac.
I've also deleted the 1Password apps from my idevices (you would think THIS would fix it, but it's not syncing to any other Dropbox location as I have verified it but I have no idea how it's getting the same info) and rebooted the idevices. I even moved the 1Password vault on Dropbox, but I'm unable to tell my iPhone (at least) where the new folder is (it keeps going to dropbox/1Password/1Password.opvault and I renamed the 1Password folder and made a new one under the Apps folder of Dropbox), but it keeps pulling the out of date info from somewhere.
1Password Version: 7.8 (macOS), 7.7.1 (iOS)
Extension Version: 7.8
OS Version: Big Sur 11.2.3
Sync Type: Dropbox