From #3956:

The above approach actually breaks tests in a different use case right now: travis-ci.org/wp-cli/entity-command/jobs/366509094#L589

The problem: The test does @require-wp-5.0 because it tests code that is only available from 5.0 onwards. However, for all tests run on WordPress latest, the tags do not get properly created, and so that test runs nevertheless and throws an error.

We need to actually resolve what version we're using when stating latest to be able to have tests for upcoming versions of WordPress not break.