Ok, I solved it:
I was flying manually, deliberately on the edge of the flight envelope, and the stall warning kicks-in.
And then the autothrust light comes on on the FCU (I didn’t see that before) and engines go into TOGA, and they stay in TOGA.
Unlike the A320, I cannot disengage the TOGA by pressing the red buttons on the side of the Thrustmaster Airbus throttle levers + physically moving them to idle. Instead I have to press the lit autothrust button on the FCU – which allows me to move the on-screen throttles again.
I don’t know if this is a bug or not, but it’s very different behaviour compared with the A320 – fly-by-wire systems notwithstanding.