I’m not sure exactly when this problem occurred, but the ECG complication no longer displays an icon on watch faces. I’ve tried deleting ECG and reinstalling it, but that had no effect. On the faces it’s on there is just a blank space and if I tap on it ECG is launched. So the complication works, but just doesn’t have an icon.
I have a Series 5 Apple Watch.
I can confirm for a corner complication on watchOS 10.1. But it’s fine for a circular complication. (I haven’t tried a long-text complication, like the bottom of Utility).
If you need it and it looks too weird not to see anything, you can add the circular complication to the three-complication widget in the Smart Stack.
The same happens with Blood Oxigen. It doesn’t matter on which position I put it, it doesn’t show.
If you tap where it is supposed to be, it works
The watchOS 10.1.1 update is out now and, while it’s major fix is to take care of excessive battery drain, I’ve read that people who had this issue with heart rate, ECG, and blood oxygen complications are reporting that it’s now fixed. (I haven’t installed it yet so I can’t verify myself.)
The update also restores the ability to put specific Shortcuts in the Smart Stack. Note that when that disappeared, I decided a better use of the limited number of Smart Stack spots was to put the Shortcuts app in the multi-app widget and call specific Shortcuts from there.
It did restore the complication icons for ECG and Blood oxygen for me. Can’t tell yet if it helped battery life, which wasn’t that bad for me.
Yes, the invisible complications icon bug seems to be fixed by the latest update, or else the process of installing the update made the problem go away temporarily, until the bug has a chance to recur. 