You are using the SB-Messaging adapter on a send port to send messages to Service Bus. You want to set the ContentType brokered message property by setting the value in the context properties of the outbound message, but the value is not honored while sending the message to Service Bus.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Note This fix requires WindowsAzure.ServiceBus package version 6.0.3 or above.
This problem is fixed in the following cumulative update for BizTalk Server:
Cumulative Update 2 for BizTalk Server 2020
Note This fix also adds a new property ContentType which can be used to configure a default value for the ContentType brokered message property on the send port.
For information about the service packs and cumulative update list for BizTalk Server, see the following article in the Microsoft Knowledge Base:
2555976 Service pack and cumulative update list for BizTalk Server
For more information about BizTalk Server hotfixes, see the following article in the Microsoft Knowledge Base:
2003907 Information about BizTalk hotfixes and Cumulative Update support