The Memory-Map apps have a "NMEA Debug" feature, which can be used to capture raw data from the GPS, or from an external NMEA data stream, for diagnostic purposes. This article explains how to use it on Android or iOS
In the Settings / GPS Settings, use the "Data Connection" button to configure your NMEA input. If you select anything other than "Location Service", then the "NMEA Debug" button is enabled.
Tap the NMEA Debug button. (The button should change to "End Debug")
You can close the GPS Settings dialog, and use the app normally, go for a short walk, or whatever while the app collects the raw NMEA data. Typically just a few minutes of debug data is sufficient.
Then, go back to the GPS Settings and button will say "End Debug". Press it and it should prompt you to Share the file. You can email it directly from there to support@memory-map.com (with details of the issue you are having) or save it to a file and look at it from another app or another device.