Same issue, using Navigraph on board map in VR.
A workaround for Navigraph is to set the Navigraph ‘scale effect’ parameter to 2.2 or more to get a readable VR window text. And set it back to 1 when in 2D.
Also the onboard map can be more readable using the Navigraph parameter ‘Use large scale map’ .
Thanks to the Navigraph team for their help on this workarounds.