As others have said, this isn't how it's supposed to work.
The best advice is to talk to GM about it, perhaps there is a problem with the car. But it's tempting to try to speculate. @Sean Nelson
's worry about the battery software not being updated is unlikely, since you have a 2019 model.
I think it might be an issue with the new more flexible charge termination feature that is new in the 2019 models. You said you charged to 80%, which is lower than the 87% “hilltop reserve” that prior years could charge to if they didn't want to charge to 100%. My understanding is that its much harder to do cell balancing when the charge is a lot less than 100%. If the batteries weren't well balanced (some above 80%, some below), that'd explain why the car thought it was doing well and then had a horrible sudden realization.
This leads to the question, when did you last charge to 100%? And would you be willing to try an experiment where you charge to 100% and then run battery down pretty low?