It makes no sense to use mag heading for any rendering on a true north Mercator projection chart.
The ownship rendering to true heading, and heading predictor vector, are displayed only if a true heading value is available.
If HDT is not actually present in any NMEA
message, we use HDM and variation (typically from WMM plugin) to estimate an effective HDT.
So, enable the WMM plugin, and all will be well.