I have the same issue.
My ATO is stable, my salt level also.
I messured over years with an IKS Aquastar and IKS platin probe without the effect of down drifting conductivity.
As I got the brand new PL4 set, I followed the calibration instructions and compared the result with the old IKS platin probe messures. It was nearly the same.
After a few days, sometimes hours - the conductivity level drops drifts down a lot.
Also I have place both probes in the same holder to test if there is an issue with the GHL probe.
The IKS probe stays stable and the GHL probe drifts down after 1-3 days. If i shake the GHL probe - both levels are the same.
There are no air bubbles, algae or something else near to the probes.
If needed, I can shoot some comparism pictures of both computers and their measurements.

Whats the cause for that? I also add an extra shielding (alluminium) for testing without any effects...