You can question the response from the fraud team. I’m sure they’d be happy to keep digging for you or could help provide some clarification on how they came to that conclusion 
You don’t need to kill the chat each time either, it’s all one big long conversation where different advisers can step in to help.
In which case, it should’ve been her card they blocked.
It’ll be interesting to see if two products arrive!
Is the time stamp for the transaction exactly the same on both yours and your wife’s personal accounts?
Should there not be some kind of unique transaction ID from the merchant (I know nothing about these things), which, if the same on both yours and your wife’s accounts would be evidence of a bug?
There should be some form of authorisation code to try and get access too? Whether these are identical or not should help get to the bottom of it
I wonder if some metadata exposed via the API could help shed some light on this. As you have access to both accounts it might be worth comparing the transaction object on both and see if any IDs match.
Assuming it isn’t user error or a display bug, it seems like somehow the card got associated with two accounts (at least temporarily - caching issue?) and the transaction got applied to both accounts and if so I’d expect some of these IDs to be identical.
The response from Monzo is disappointing. The auto-fill hypothesis could’ve easily been disproved on their end by checking both accounts. If the transaction indeed appears on both then something is seriously wrong and it wasn’t auto-fill.
For that reason, if further discussion with support doesn’t get anywhere, I’d suggest registering a formal complaint. Even if there turns out to be a reasonable explanation, a potential bug like this deserves to be investigated properly in case it turns out there is something that needs to be fixed lest it happen again to other customers.
I am usually very much against raising formal complaints about a non-financial issue like this (typically I’d raise a formal complaint only about a finance-related issue, like wrong interest being charged, money missing, etc) This seems to be more like a technical issue (maybe just a display issue) that resolved itself. However, with Monzo starting to pull some BS and act in bad faith when it comes to support (hiding the chat button and cutting corners with support quality) I guess it’s fair to fight fire with fire.
My view is that if COps are trying to brush you off instead of escalating the issue to a tech team, then raising a complaint is fair play. I would disagree with you that it’s a non-financial issue because to me, if something that should be happening on one account is showing as happening on another account, that is a financial issue.
It’s also potentially a data protection issue (or something akin to); if person A is making the transaction on their personal account because they don’t want person B to see it, but person B then ends up seeing it because of [whatever issue or combination thereof] that went wrong here, that’s a big problem.
This has kinda happened to me too! An attempt by a phone company I’m no longer with, to take money from my Monzo account keeps getting made. Thankfully declined as I only using my Monzo as emergency fund whilst I’m abroad BUT I never used Monzo for my phone bill but still it seems to have created a place for it under ‘bills’. There’s no option to stop it either whilst I try to talk to the phone company directly (another hassle all together) which isn’t helpful, so they’ve blocked the card. I can’t have a new one sent as I’m no longer in the UK and it would cost me to have one sent to NZ. So I guess it’s staying blocked until I get through to the phone company.