[BUG] "All flags" shows 100% coverage, but all flags combined do not have 100% coverage #1961

Describe the bug
On propcache, we see coverage under the default "All flags" which does not exist even when every single flag is selected manually. I don't understand how that can be possible.

Additionally, in this example, if I just select a single flag like OS-Linux, then it shows that the only missing coverage is a partial. But, it says that both the return inside that condition and the code after that condition both have coverage, which again shouldn't be possible.

To Reproduce
https://app.codecov.io/gh/aio-libs/propcache/pull/251/blob/tests/conftest.py?dropdown=coverage

Select every flag and the coverage drops from 100%.

Expected behavior
"All flags" and selecting every flag to produce the same output.