I just tried entering a GM vehicle on a Form 8936, purchased 8/15/2019 and I didn't get the 50% phaseout either - it's allowing 100%. I tried all the other GM "Makes" the program looks for and none of them worked. Tesla works, though.
I also noticed that if a second vehicle is entered and it's a GM Make then the phaseout is computed. Hmmm.. that's a bug fer shur. I'll get that reported.
For the phaseout the "Make" entries it looks for are (case insensitive):
Buick, Cadillac, Chevrolet, Chevy, General Motors, GM, GMC, Tesla.
