Default output from UM982 for heading
#AGRICA,81,GPS,FINE,2274,292528000,0,0,18,27;GNSS,232,23,8,9,9,15,10,5,4,7,5,6,-0.9135,1.4475,-0.0400,0.0541,0.0978,0.1225,0.5867,-0.4194,0.0000,0.003,0.001,0.002,-0.002,0.051,0.040,0.065,50.95425087438,6.89550221116,48.3615,3996923.3714,483362.5189,4930414.1171,0.0550,0.0981,0.1250,0.1175,0.0941,0.0748,50.95425908545,6.89548161083,95.3171,0.00000000000,0.00000000000,0.0000,292528000,1.000,46.991943,46.915642,0.000000,0.000000,6,0,0,0*c64c9770
Px4 required format:
#UNIHEADINGA,97,GPS,FINE,2190,365174000,0,0,18,12;INSUFFICIENT_OBS,NONE,0.000 0,0.0000,0.0000,0.0000,0.0000,0.0000,"",0,0,0,0,0,00,0,0*ee072604
I already sent a mail to Holybro support regarding the issue and requested them to update it.
If not we can add the work around to the px4 docs for now.