I got an eTrex 30 for myself for Christmas; and I'm suffering the same problem -- If you enable MyTrails; it does a complete whiteout of the map; totally obscuring the underlying garmin 24K map I have on my device.
I uninstalled MyTrails; and then reinstalled it so that it would appear after the Garmin 24K map in the device's "SELECT MAP" menu; previously, it had been before -- to see if this would fix the problem. No Go.
My position is in the Washington DC\Maryland region if you need that to check transparency.
The same unit also correctly displays transparency for trail100 in the same area.
I uninstalled MyTrails; and then reinstalled it so that it would appear after the Garmin 24K map in the device's "SELECT MAP" menu; previously, it had been before -- to see if this would fix the problem. No Go.

My position is in the Washington DC\Maryland region if you need that to check transparency.
The same unit also correctly displays transparency for trail100 in the same area.