While this behaviour is understandable, it would be more useful to show the last heading. As we cannot test with all receivers in all modes, you may want to review the output of your receiver to make sure that what you can get is suitable for your application. The course and speed mode of the display is selected by a jumper on the board which is sensed each time a new RMC sentence is being read. In fact, the ability to drive multiple serial receivers with one serial driver depends on the particular drivers and receivers you are using, cabling details, and noise in the environment. While success at this trick is not guaranteed in all situations and configurations, there is a very good chance that you can make it work in your situation.
Receivers output their data under some serial line signalling standard.
The amount of current source or sink capability is also important. The serial specifications require a driver to provide more than enough drive for one input receiver at some line length and speed. In practice, drivers typcially have a safety margin in their capability to drive the line. This safety margin helps support multiple line receivers. Cabling is a factor here, and the best bet is to keep cables short and take reasonable care in cleanliness.