I had a couple emails back and forth with Plastiq, and they admit that they're authorizing as a cash advance but then putting it through as a payment. So it's Plastiq, not Chase that's doing this here, and so thus presumably anywhere else you've run into it it's also being done by the merchant, not by Chase. (Now, whether Chase is recommending to them that they do that, that I don't know, but it's the merchant who is actually doing it.)

But they also say that this cash advance authoriization garbage is temporary (so temporary that that's why they're not necessarily going to work on documenting it any better, despite admitting that the 20% stuff is not good documentation). What is going to happen after this "temporary" phase ends, I don't know, but I don't think it's worth running experiments on something that's temporary, if you don't need a temporary way around it.

In my case, I gave up on using the Marriott personal Visa because it turns out I'm going to meet the minimum spend easier than I thought even without this payment.

So I'm also now using the MasterPass promo (on a card that gives me some extra points each month from a retention promo), since I have to make the payment before I could apply for and get any other MasterCard or business Visa that gave a signup bonus with minimum spend; (They already told me that this payee doesn't work with Amex currently, even though it worked with Amex a year ago, and that's why Amex is not possible either.)