I took a look at a couple of TAIP specs (including one from the sierra website). I don't see anything about handling zero speed differently for heading. The TAIP protocol is different from NMEA in that it's positional rather than comma separated. I.e. when NMEA doesn't know the heading that field is left blank, but TAIP doesn't really have a choice but to put in something. I suspect that other models preserve the prior heading when speed goes to zero, but I don't know that for fact. I do understand that unless there has been some movement of the GPS that they can't determine Heading, i.e. Heading is derived within the device using multiple recent positions, there isn't a magnetic compass in a GPS device.

Do those new devices have the ability to switch to NMEA? That might be one solution. You can also file an SR and ask for a CR, but no promises on if or when there might be a code change -- it would be non-trivial because the various sentences get quickly collapsed to a common internal format in the MPS GPS library so it may be difficult to know when to ignore the 0 heading.