I have ISO stock options as part of my employment in California, USA. In the middle of 2019 I had to leave the USA for family reasons. In 2020 I wasn’t in the US (became non-resident alien for tax purposes). In the beginning of 2020 I executed all my stock options, most of them vested while I was resident in California in 2016-2019. Since the options vested while I was in California, I’ll need to pay AMT tax in 2020 after after executing stock options. Are the stocks that I acquired after executing stock options subject to US taxes if I sell them later while being non-resident alien?
Assuming that you hold the shares acquired with the exercise of your incentive stock options for more than a year after the exercise and two years after the grant, any further gain (or loss) - measured by the difference between the sale price of the shares and the amount paid for them on exercise ("strike price" or "exercise price") - should be considered a long term capital gain (or loss). As a nonresident alien, that gain should be exempt for US Federal income tax (don't know about California specifically).