Both milestone and devel are unreleased versions of ansible-core (2.20 at the time this issue was reported), and there is no practical point in keeping both for testing purposes.

Moreover, they currently disagree about sanity tests that lead to the issue in the CI for repositories that use the current state of CI.

Some discussion was in Matrix, I want to use as the reference comments by @felixfontein, actually this one and this one.

So, the suggestion here is to remove milestone from the matrix and keep only devel