Thanks for reporting this. Yes, it looks like this only occurs when switching from a GPS unit to the "No GPS" option; it shouldn't occur if loading into the aircraft with the "No GPS" selected by default.
As you have mentioned, the workaround for now will be to ensure VLOC is selected on the GPS prior to selecting the "No GPS" option. Another workaround will be to select the "No GPS" option on the EFB, then restart the flight so that the "No GPS" option is selected by default.
We have logged this on our internal bug tracker, and we will investigate further ahead of the next update to the PA-38 Tomahawk.
Mark - Just Flight