Thanks David. I was following the instructions in the Connect to ServicePlatform with JSON configuration, which is a global configuration that affected all endpoints. Pulling the JSON Heartbeats code and configuring the endpoints separately as you recommended is definitely the way to resolve this issue.
Unfortunately, the issue in Configure Quartz.NET to Trigger Sending Batches of Messages Without Heartbeat Errors still occurs (albeit now on the receiving endpoint), so I had to pull Heartbeats entirely (as you suggested might be necessary). C’est la vie! Appreciate all your help with this.