Not sure how much you know?
If the salinity keeps rising its most likely evaporation. Top up with freshwater. Normally RO
When I transitioned my puffer fish from shop to home it was suggested to me to keep the salinity within 0.001 per day and increase gradually until the desired level was reached. This allows the bacteria to adjust with the salinity.
The sudden shift could be causing osmotic shock/stress.
Regular top ups should help keep salinity constant. With the puffer fish I usually add RO
twice per week and change matched water on the weekend. Only adding as much as has evaporated.
If you started at 1.026 and finished at 1.023 your clean water potentially was only 1.017 perhaps less? That's a considerable difference in Grammes per litre of salt between tank water and new water.