This is an interesting observation: I responded to someone's public post on a Mastodon (glitch) server as a private message (I have a toggle in my Compose interface that allows me to control that). That message went out fine and they received it, although I'm not sure what the UI looked like.

Their response #ActivityPub message from Mastodon however was unusable. I found it in my database, but the to and cc fields were empty.

{
  "cc": [],
  "to": [],

  "type": "Create",
...
}

I suppose I could still pull these up for the UI using the inReplyTo field. But blanking out the address fields seems like a weird choice for #Mastodon.