While using a vector brush, it doesn't matter which one, if I try to change the "pressure" using the drag handles on the Stroke slide-out, the handles merely pop right back into the top-right and top-left corners, with no apparent change to the pressure setting. If I change the "controller" to "pressure" within the contextual menu at the bottom of the screen while using a vector brush, it causes the iPad to freeze completely.
The only way to exit is to lock the iPad using the power button. The screen comes back to the frozen Designer screen, but it now gives me the ability to swipe-up. Once I swipe up, Designer closes itself.
iPad Pro 11, iPadOS 13.3, Apple Pencil 2, Designer for iPad 1.7.3