UPDATE:
Situation resolved. I jumped the gun and own the responsibility.
Plastiq apparently sent an email to the "dummy" account requesting additional verification. They put a 0.79 charge through on the card and needed confirmation to process the payment. The emails slipped past me and I never responded; hence the payment being declined.
Long story short: it's my fault!
I've never been asked for verification prior and was not expecting it.
According to the service rep. that I just spoke with, this can be requested with any payment, which I find somewhat troubling and unnecessary. Thankfully, I've not been subject to it via my own account as yet. If that changes anytime soon, I may just bid farewell to their services as I don't want to jump through hoops to pay rent.
Btw, I asked whether they would consider reinstating the FFD on my primary acct as a one-time courtesy (with no expectations whatsoever!) given the inconvenience and potential impact of this situation (ie. incurring fees and penalties for late rent) and needless to say, they would not.
I'm sorry if I scared anyone with my earlier posts. I was trying to be helpful by sharing my experience, as it unfolded. Perhaps I should have waited until I had all the facts.
If Plastiq is going to start requiring verification for payments going forward, further slowing down the process, I will cut my ties. I'm willing to occasionally submit to a 2% fee for convenience and efficiency when meeting min. spend on new cards or topping point balances, but not if they start complicating the payment process. Can I ask how many others have experienced issues with payment verification requests? I was under the impression that such verification was a 'one-time' thing, but according to the rep I chatted with, it can apply to any payment at any time.