Artificial seawater is problematic because the salts in artificial seawater do not completely dissolve leaving you with a solution that does not completely match the ionic concentration of seawater. This is true for synthetic blends (made for aquariums) or ones made from drying natural seawater. The different ionic concentration leads to drift and potential offsets in the pH sensor. This leads to an inaccurate K0 and initial drift in the external reference of the pH sensor, the internal reference electrode should be unaffected because it separated from the seawater by a saturated solution of KCl gel. This occurs because the pH sensor external reference electrode is a solid state AgCl electrode which is in direct contact with the seawater. A unconditioned Ag/Cl external electrode K0 typically changes by 3mV (60mpH) when exposed to natural seawater for the first time. The challenge here is without knowing the ionic composition of the artificial seawater it is difficult to determine exactly how long the drift will occur or if there will be a permanent offset. This is why we recommend to only use natural seawater.
When the Ag/AgCl is first installed in the sensor it is pure Ag/AgCl, but when it is exposed to seawater it reacts with the ions (Br- mostly, but there are other ions too) in seawater which changes its ionic composition and its standard potential. The standard potential is the K0 coefficient which is provided to the customer and the manufacturer the sensors in seawater for ~3 days in natural seawater to ensure the external electrode is conditioned to seawater prior to K0 calibration in seawater baths. If the sensor is then put into artificial seawater with the incorrect ionic compositions, you risk deconditioning the external reference which could lead to inaccurate measurements and drift in the sensor when first deployed. The sensor should recondition to seawater after deployment. The reconditioning can take 3 days to 2 weeks, however permanent offsets can remain even after the external reference has stabilized.
However, if you cannot obtain natural seawater on a regular basis. You could obtain some natural seawater when the pH sensor is recovered or deployed. The seawater could be taken back to your lab, filtered and stored in a dark container in a cool and dark place for a couple months. Just be sure to filter the seawater again before you fill the wet cap to store the pH sensor for long periods of time.