GNSS module direct grounding to ESP32 increase sensitivity
I suspect the ground wire connecting the two modules being &quot;thicker&quot; might have similar results. Based on what you are telling us, the only difference between the two circumstances is that the more favorable one has &quot;less resistance between the two grounds.&quot; More resistance between the two grounds will generally result in the voltage on the two modules being more different than you might guess, and notably the voltage the GPS module is getting might be effectively reduced relative to its local GND, making it perform less well.
Another possibility is that current flowing through the ground wire connecting the two modules is generating radiated emission that interferes with the GPS receiver's sensitivity (signal-to-noise ratio), so the circumstance with the shorter connection where this current flows minimizes that effect.