Apparently, how each device identifies its own coordinates depends heavily on the precision level set for the primary channel. So, if you are on an expedition or require multiple nodes to synchronise high-precision coordinates, the only current method appears to be configuring your private channel as the primary channel.
After that, create a LongFast channel and manually set the frequency slot, since slot 0 will automatically change if you modify the primary channel. In my case, I had to set it to frequency slot 4 on SG_923.
TLDR: Meshtastic uses the coordinates of your primary channel as a reference. If you are new to Meshtastic and suspect your GPS is faulty, it is likely due to the precision level setting