VTG having the correct speed but the wrong direction is something I have seen in the past. It is the biggest concern when using VTG without GGA for comparison. When we evaluated the issue we couldn’t correlate when this happened with any of the GPS quality indicators. When using VTG it is recommended that the boat speed be greater than 0.24 m/s (C.R. Wagner, D.S. Mueller / Journal of Hydrology 401 (2011) 250–258) but I don’t think that guarantees this won’t happen.