Yaw was never affected but magYaw was. Yaw is a value computed by the Flight Controller that represents what the FC thinks is the heading. MagYaw is computed from magnetometer and attitude data - it's independent of the Yaw value. Yaw is used by the FC for navigational purposes. Yaw is subject to small random errors and left uncorrected will slowly drift from the correct value. MagYaw is used by the FC to make the necessary minor corrections to the Yaw value.
If the P4PV2 had been powered up at the launch site Yaw would have been initialized to the incorrect magYaw value. This would have lead to erratic flight and probably a fly away. But, it wasn't powered up at the launch site and Yaw wasn't then set to the incorrect magYaw value.
Normally what happens is that the FC tries to fix a Yaw/magYaw separation occurring before flight by changing the Yaw value to agree with the magYaw value. In this instance the Yaw value would have been set to an incorrect magYaw value. Fortunately, that didn't happen here and the Yaw value wasn't compromised. At launch Yaw was correct. The erratic flight was not caused by the fact that the launch site was geomagnetically distorted.
The reason I suspect that the cause was due to poor GPS data can be seen here.
View attachment 113790
The blue plot is the Latitude computed by the FC and shows the P4PV2 moving south. This in incorrect since the P4PV2 was moving north. But this isn't totally conclusive since the FC uses other data besides actual GPS data to compute this value for Latitude. More significantly is that the raw GPS data (red) also shows the P4PV2 moving south.