I've noticed that my MX only charges to 88% when set to 90%. If i log out of the app and go over to the official tesla app, it starts charging the last 2%.
If you wouldn't mind, I have a few questions to help narrow it to down.
Are you saying you set the charge limit to 90% using Dashboard for Tesla but you witnessed the car only charges to 88% before charging says its complete and charging stops?
If you use the official Tesla app and set the charge limit to 90% then it does charge to 90% before charging says its complete and charging stops?
What firmware is your Tesla running? If it's a new one there might be changes to the API that are the culprit.
After setting the charge limit and refreshing the charge screen in the Dashboard for Tesla app, does the text immediately below the charge limit button show the set charge limit as 90% / STD? Or does the text say 88%?
Are you using the Smart Charging feature?
Are you using any Automatic Behaviors that involve stopping or starting charging?
Do you have a scheduled charge setup in the vehicle's settings?
Are you in a cold climate where the battery heater could play a factor?
Do you start the HVAC running after charge completion?
Is the reported rated range (ie, 212 miles @ 90%) the same when you use both Dashboard for Tesla and the official Tesla app to set the charge limit to 90% when charging is complete?
For the record, the only thing in Dashboard for Tesla that you can setup to automatically stop or start a charge is Smart Charging and Automatic Behaviors.
Opening the official Tesla app should not automatically start a charge either as far as I know, perhaps on newer firmwares the charging could still be ongoing at 88% and the API is simply reporting it as charge completed while the final 2% secretly finished up in the background.