iv noticed exatctly when its happening. i set a key on the curves layer, move ahead 1 or 2 frames, curves magically converts to pencil mode. step back 2 frames (not undo, just scrub), and its back to bezier mode.
further playing around showed me that if im between 2 keys, its in pencil mode. if im on a key, its in bezier mode. when i pass the last key, it stays in bezier mode. so the solution i guess is to copy/paste a key to where i want to make the new key, and adjust.
perhaps this is normal ae behaviour? i dont recall this happening before, but maybe it was.