Thanks for the thorough reply.
But the thing is, and according to the documentation (and thus the reason why I rely on it), that
update_row() should return true or false based on success or failure of the function (https://www.advancedcustomfields.com/resources/update_row/).
And this used to work!
After the latest update of ACF Pro, this is not the case anymore.
I also submitted a support ticket, and I’ll expect a reply after the weekend.