Federation: comments not syncing

I'm experimenting with hosting my own instance. It seems like things are working, e.g. the steps in the troubleshooting doc work, http routing seems correct, etc. I can subscribe to other instance's communities and see the posts. However, comments are not syncing at all not even new ones. I suspected it might be a lemmy.ml problem, but I also get this behavior from other instances as well. I am running 0.17.3. How do I debug this, or is it the expected behavior (new to Lemmy, of course).