On iPhone, the limitation appears to be the Panasonic Image App itself and/or iOS background restrictions. Based on the manual text quoted in the community answer, GPS recording stops if you press the Home or power/lock button while location logging is active. In practice, that means logging is reliable only while the app stays open in the foreground and the phone remains unlocked.

The reported test supports this: after starting logging and locking the iPhone for 10 minutes, the log counter only increased for roughly the first few minutes, then effectively stopped. So if your photos all map to just a few points, it’s likely because the app wasn’t continuously recording once the phone was locked or sent to the background.

To get the best accuracy, keep Image App open and active during shooting, and avoid locking the phone. If you need continuous background location tracking, this app on iOS may simply not be suitable for that workflow.