Problems using an external GPS

I tried using Gaia GPS with an external GPS (Holux M-241) using Bluetooth GPS as the intermediary application to provide a 'mock location'. This set up works fine with other android mapping apps. With Gaia on the first day I (samsung s21) and a collegue (samsung s10) both had our positions located approximately 750m to the NW of our true location. This was with all the usual position assists enabled. The position icon came back to the correct location after disabling the external GPS connection. On the second day I turned off all location assists and enable GPS only. I started recording a track without the external GPS. On connecting the external GPS the position icon stayed in the one location while I moved off. On disconnection the icon came to my true location and filled the missing track with a straight line from one point to the other. Essentially it appeared to stop receiving location data .

I read somewhere that Gaia GPS will switch between devices to get the 'best' location. This could possibly be causing the problem.