Got a reply from MS:
'
That’s definitely a different problem from what we saw with the Simplo batteries. I’ll send it back up to the engineering team.
'
For the people trying to track this down.
I have the SIMPLO batteries.
BEFORE the update I had all of these problems.
Could not run without a cord.
Surface would shut off even with the cord connected if the battery was at about 2%.
(The battery ran down if I was running my i7 at full speed because it disables charging)
Removing cord would kill the surface.
Very low battery capacity (about 10 minutes).
AFTER the update, all of my problems were resolved.
Hey could you please specify which update corrected your issues?
I have the EXACT same problem as you with a SIMPLO battery and so far no dice...