Microsoft just brought an update to Surface Duo 2. While third-party notifications being supported through the Glance Bar and automatic pairing of the Surface Slim Pen 2 were what’s new, pairing to the original Surface Pen has been cancelled.
A Reddit user inputs "Looks like they removed the ability to pair with regular surface pen- only the slim pen 2 can be paired for using button-click shortcuts.”
It was also known that a third-party styles called the Uogic Pen which could run on the Duo and related devices, could also work with the Duo and Duo 2 now, thus implying that this alteration only affected the original Surface Pen.
Though this original pen is still compatible for Duo and Duo 2 with inking being supported as well, actions like launching an app or taking screenshots cannot be done now.
This original pen has shown glitches occasionally and looks like Microsoft removed this feature to prevent that issue for now, although not anything being officially reported by the company.