Unsafe AIS oversight

[W5PAD]

I've found a specific combination of Class B AIS transceiver and vessel speeds which do not show a vessel's icon on some MFDs.
The AIS unit I discovered the issue with is a B&G NAIS500. I'm sure all Navico units would be the same. As far as I can tell, the actual manufacturer for Navico units is Alltech Marine Electronics Corp (AMEC). I would also presume all AMEC units regardless of whose name is on it would present the same issue. The MFDs I'm aware of that play an integral part in this issue are Standard Horizon (model CP300I) and Garmin (model 740S).
The issue occurs when the suspect AIS measures a low or zero SOG, whereupon it sets COG as invalid. The suspect MFDs do not display an icon without a valid COG. However, the AIS vessel is listed in the AIS List. So the MFD knows the vessel's location but does not display the icon on the chart. When the AIS vessel's SOG increases, the COG becomes valid and the icon is then displayed on the chart.
Looking further into this issue I found other AIS units set COG as valid when SOG is low or zero. So these AIS units' icon will be displayed on the suspected MFDs. Also, other MFDs (verified on B&G Zeus Touch and Raymarine A series) will display an icon even when COG is invalid.
For my setup, I also found that if Heading is sent to the NAIS500 it will transmit this parameter in AIS Message 18 and the suspected MFDs will then display an icon. This lead me to conclude that the suspected MFDs need at minimum either a valid COG or valid Heading to display the icon. If both are invalid then the icon is not displayed. Heading can be sent via NMEA0183 or NMEA2000, I tested both with success.
If you have a suspected AIS and want to be seen on all MFDs but do not want to wait for firmware updates, then look into sending Heading to your AIS.
Has anyone else noticed this issue? Was a particular vessel's icon display sometimes and not at other times? Could this difference be attributed to being underway versus anchored or moored? Do you have a Navico, McMurdo, or AMEC AIS and someone didn't see you on their MFD? Do you have a Garmin, Standard Horizon, or other MFD and didn't see a known AIS vessel on your MFD? If known, who produced the AIS that wasn't displayed?