As the subject states. I have a colleguage asking about this, and found that task comments are only published as e-mails and to the group conversation, not in Teams. Bug, lack of functionality or wrong settings?
Lack of functionality for now. Hopefully much tighter integration between Teams and Planner is coming.
For now, you could create various workarounds for this, but none would be ideal. Manually you could use the "email a channel" feature. For more automated, you could look to use Microsoft Flow.