So,
(Q1) how does the Strava device broadcast (or GPS satellite pick up) it's location while it is meters under ground & water?
(A1) It doesn't. Basic knowledge of how GPS works, proves that.
(Q2) Why would the Strava device be locatable under ground AND water, but is never picked up when just under ground?
(A2) Once again, it can't - common logic can assist here.
Correct answer to Strava tracks between islands - The Strava locations & tracks being discussed here are on Strava dudes riding on BOATS (or in some cases - swimming/diving). All being pretty big vacation activities in the Bahamas.