Let me make sure I understand the symptoms.

In filter mode, pressure drops from 14-15 PSI down to 12-13 PSI after 30 min and flow rate is reduced. Does this repeat when you turn the pump off then on again? You gave a range of PSI, does this mean the needle is moving or is the pressure between the two marks?

Pressure and flow rate remain the same in recirculate mode. Have you run this for at least 30 min too?

No air in the pump basket?

If the problem was in the filter, pressure would increase with decreasing flow rate. However, since the filter pressure decreases with decreasing flow rate, the problem must be before the filter gauge. But since it does not occur in recirculate, it cannot be anything before the filter.

So assuming the filter gauge is correct, the only thing I can think of is that there is something wrong with the multi-port valve. Perhaps a torn or misaligned spider gasket. When in filter mode, is any water coming out of the backwash line?