Painting in video timeline not working

Hi,

I've been doing some frame to frame animation using the Photoshop video timeline and all of a sudden it won't let me paint on the video frames. I can paint on a static layer but on a video layer it becomes blocky or just doesn't show.
I'm using Photoshop 24.7 but I've tried it in Photoshop Beta and that works fine.
I updated my Nvidia Studio driver  (V 536.99) and rebooted and there.s no change.
My system is Windows 10, 128GB Ram, RTX 3090Ti GPU.

Any ideas?

Thanks!