To reproduce, start a new document in Photoshop - hit B to go to brush tool.
Then press and hold CTRL+ALT on keyboard, followed by pressing and dragging pen on ipad pro horizontally (shortcut in Photoshop CC for increasing/decreasing brush size).
The brush size will incrase/decrease correctly, but when you release CTRL+ALT the active tool will lock on to brush resize. If you now try to draw on ipad pro it will still increase/decrease brush size instead of painting.
This only seems to happen IF you release the modifier keys before releasing the press down on your ipad pen
This also happens sometimes when using only the ALT modifier for picking colors, using astropad to pick color in ipad will make photoshop think that ALT key is still pressed - keeping the colorpicker tool active..
This is a huge bug that makes astropad less useful for me as an artist to paint with. I do hope you guys can fix it soon!
EDIT seems a workaround until fix is released is to make sure you release pen from surface BEFORE releasing modifier keys (ALT / CTRL).