Strange.
However it rreminds me of a charted 757 we had for few weeks. Always around 1500' AGL on autopilot the flight directors dissapeared. As soon as autopilot was disengaged the came back in.
The reason for this, I am told, is the reason that somewhere (in the UK?) there is a rule that the flight director can not use the same information as the flying autopilot, i.e. from the same computer.
At 1500' AGL on a 757 all the autpilots kick in for LAND 3 and therefore, the flightdirectors dissapear.