I should mention this has been creeping in for some time – it started in the emission setting of the particle emitters – so if you wanted to do a shot of 1 in the PMatterWaves and then keyframe it to 0 on the next frame you can’t. (for a true “shot” – why it can’t automatically do what the name of the slot is I’ll never know) So now this seems to have infected the Constant node as well now.
I should also mention the work around, in case someone comes upon this in a search. You key frame your first frame the normal way – that works. Then go into your dope sheet or Fcurve time line and advance to the next keyframe position and add that second keyframe there in the dope sheet or Fcurve editor, and then set that keyframe to what ever value you like.