It appears that the current geolocation field implementation does not allow entering custom coordinates. Due to the underlying code using the PlaceService.QueryPlaces API endpoint from Google Maps it tries to return a named location (e.g. the nearest inhabited area). For example, when entering “69.418017 -150.309004” (located in the wilderness of Alaska) it returns “69.0579 -152.8628”, which corresponds to “North Slope, AK, USA”; obviously, way off. It would be best if certain input patterns resulted in directly parsing and setting the lat/long values (e.g. when the user enters two float values separated by a space or a comma as in the example I gave).
Edit: As a workaround, I published a plugin called ‘Manual geolocation’ that lets me do exactly what I need.