ATR 72 Captain reported encountering severe inflight icing and as the flight crew followed the QRH and selected MCT power mode; the Flight Director decreased the airspeed to an unsafe range.

Narrative

ATR 72 encountered severe ice. ATC recommended higher and granted a block to attempt to climb out of the icing conditions. Following QRH pilots selected Maximum Continuous Thrust (MCT) power mode and IAS on the Flight Director (FD).In an uncommanded and inappropriate computer action the FD moved the magenta airspeed bug from 170 knots to 156 knots; then to the high 140s. The computer was reporting DEGRADED PERFORMANCE; all anti ice equipment was on; MCT was selected; props were at 100%. It appears to be a programming error that under these conditions the speed bug would self select a too low airspeed rather than the icing bug plus 10 or plus 30 that are appropriate responses.Pilots flew by hand and set manual (cyan color) speed bug to an appropriate speed instead of following the magenta (automatic) speed bug.Can someone please check in with ATR to address this dangerous automation defect. IAS mode should not commanded slower than 170 in these conditions.