German umlauts in the Subject are replaced by a question mark

Symptoms 

You send an email message that contains the German umlaut character in the Subject line. The Exchange Transport Rule (ETR) prepends the subject (for example, [EXTERNAL]) to the existing Subject text of the message. 

In this situation, the umlaut in the Subject line is replaced by a question mark (?). 

Resolution 

To fix this issue, install the following cumulative update: 

• Cumulative Update 15 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019