Soldier experience is not working well, sometime requiring several clicks to allow a new class selection, the weird thing is that several ‘window’ of class selection will open, usually on after a different soldier experience is selected for upgrade. Eventually, the point spend (new exp on the second soldier + new class) might exceed the total phoenix point available showing a negative value of point available.
I can testify, you can spend more then you have and then you get -
Dont worry it will be deducted from your next “earnings” - like SOS credit on prepaid.
Its just another miracle of magic SP and Crazy skills system, instead of soldier natural progression, partially implemented in XCom 1994 (or best seen in Civ6 for civs)