We have multiple streaming pipelines (using state + timers) that, after upgrading to 2.24, exhibited very strange watermark behavior. The watermark on some stateful DoFns would advance to the end of the first window, and then get stuck there forever, even preventing the job from draining.
I was able to track the problem down to https://github.com/apache/beam/commit/88acc5267f759d81e9836a9db17b9e0ee521c785. After revering it, the behavior went back to normal. It looks like its possible in that commit that watermark holds for some timers aren't being cleared.