Hi, thanks for sharing this issue.
ExcludeFromCodeCoverage doesn't work here because at a physical level, the attribute doesn't get applied to the compiler-generated IL structure that comprises the async code block.
It is possible for us to fix this. However, we can't find a way of doing so that doesn't massively impact performance of the instrumentation .. such that the fix is worse than the problem.
VSTest has the same issue. I'm not aware of this attribute working in such a scenario with any other runner either.
I recommend using
code coverage suppression comments instead.