Thanks!
Well, neither the stab nor the leading edge of the flaps are heated, so ice with always form there when in icing conditions.
I've never noticed this effect on the cfm bus, but here it's a bit startling. Had my WTF 😳 moment that night. Also it feels like the aircraft wants to roll left and right, like almost all roll stability is suddenly gone.
Flying 5 kts faster didn't get us out of it, didn't try higher as a slippery runway was so be expected.