Watch Blood O2 disappeared

My Apple Watch 6 used to work perfectly with the Blood O2 app. Now when I open it I get the “Blood O2”title and a blank screen. No other display or start button (picture attached). The iPhone 13 Pro Max Watch app shows Blood O2 as present and turned on.

Any ideas how I can solve this? As it’s part of the Watch system I can’t just delete and re-install.

Thanks