Oh, just realized both of your comments are probably about the vault puzzle, not the paywalled bonus puzzle. If so, I hope my other response clarifies things. (And interesting that it almost works with the diagonal restriction! I guess that's essentially how two-cell cages work with the vault rule.)

I'm pretty confident that all of my puzzles are unique (but mistakes can happen, obviously). Do you have a screenshot showing the other solution and/or the point where it's unresolvable? I know it's paywalled, but I don't mind if you post it here (for this purpose, at least 😀)

A digit in a cage can't be in a cell that's orthogonally adjacent to *any cell in that cage*. So, for example, because 1 is in the 6-cage in r5c2, it can't be adjacent to it in r6c4.

Thanks! Yeah, it definitely came from trying a similar setup and seeing what it did to 1s and 9s. In fact, you can see the original idea here:
logic-masters.de/Raetselporta...

Sorry, just seeing this now! If you can post a screenshot, I'd be happy to give you a hint (and you can always check Rangsk's video 😀)