Now that is strange. TBH, I had not tried buying something right after getting declined at MO, instead I called in to get my card "unlocked" and put my KYC on it.
Bought a fruit at self-checkout and MO'd out next day (but left some scratch on the card).
I suspect (((they))) are watching for full cashout of loaded amount, but it is strange how sporadic this appears to be. No idea if adding your KYC to card will help, but that's clearly not in the interest of time.

I'm still staying away from USB as it's just a pain when there's other options in the game.