My recollection of the BAC 1-11 was no L.E. slats, but I think it did have flap asymmetry cut out. Big flaps had quite an effect on stall speed - all favourable - so I still can’t see why an operator would prohibit selecting them in a turn unless they expected to take a missile strike at some time.
Years ago I experienced flap asymmetry in a DC3. As full flap was selected there was a nasty cracking noise and the old beast rolled very rapidly. Although it was controllable, we guessed one side had fully retracted, while the other side had fully extended. We took a punt and selected flaps up. This stopped the rolling tendency. DC 3 flaps don’t affect the stall speed by much, but they sure make great spoilers!
The problem was a hydraulic actuater rod had sheared its end fitting.