Thanks for the comments and suggestion, 1P_PeterG - I'm not too worried about the stranded data in the 8.x beta on my MacPro. I've already gone ahead and performed password resets for the services impacted, and re-entered the other data that was missing into the 7.x version.
The core issue, however, is one that I did report some months ago.
Essentially, "syncing" on macOS isn't working right with 8.x.
I can probably find the thread if needed, but, the problem I've been having is that 8.x would regularly not send, or receive, updates from 1Password.com.
For example, if I used my iPhone and added a new item, it would show up fine on the 1Password.com website, and my iPad, but not show up at all on macOS 8.x beta. I could wait minutes or hours, no change.
However, if I made a minor modification to an existing item on 8.x on my MacPro, for example, adding a Note to an existing item, it would kickstart something and the missing item from the iPhone would appear shortly thereafter.
When things broke the other week and I reverted to 7.x, I'd already checked 1Password.com to see if the data from macOS was being pushed up, and, nope, it never made it there for the items that are now gone. It was after that point that I had hoped that signing out, and back in, on macOS, would work. Which is what left me in an impossible to sign-in state.
The emergency kit would not work either, macOS 1P 8.x would not accept my username/secret key/Master Password. My gut feeling was that this was somehow related to the fact that I had both Authy AND YubiKey set up for MFA on signing in to 1P.
So, I could have visited this forum from my iPhone or iPad, but, I had work to get on with and the priority was getting 1P working on macOS. So far, 7.x is doing just fine, and I really, REALLY missed the ease with which I can move items around between vaults, delete multiple items, and several other things that 8.x just did not support (and for which the UI was frustrating).
Alas, I don't feel comfortable with the 8.x beta now and trying to pin down what caused this, as my reliance on 1Password is just too great, and being locked out isn't something I can risk again. I am getting a new laptop soon, so, I may throw the beta on there as it won't be a critical system.