Haptic but no sound for Messages

Every alert I get on my watch gives both haptic feedback and a gentle ping sound, except for Messages. When an iMessage or text message arrives, I get the haptic feedback, but no sound. The Messages app is the only one affected by this. I've gone through all the settings but I can't find anything related to it.

I've found other threads similar to this question, but the answers relate to older versions of watchOS so don't apply.

Any thoughts?