Hi,
I have had a similar problem. I don't know if it's a glitch or not. For me, the Datum plane would reverse it's vector if it was Parallel to a face/plane and through an edge.
The fix I came up with was to make the Datum parallel to another datum Plane (vector control) and through a Datum Axis. The Datum Axis direction is driven by a Datum plane also, but on the edge in question. (Perpendicular to Datum/through point) Now, when I make updates to my model I don't worry so much about the datums "flipping" and causing sketches to fail.
I would suggest you make sure that the Datum is either relative to another (Fixed) Datum or from a face you are sure will always be there on future edits.
Bubbers.