All softeners, backwashed or regenerated filters have a SFR (service flow rate); maximum gpm that if exceeded prevents the softener or filter from removing all of what they are supposed to. The hardness etc, that gets through the softener or filter is called leakage.
The SFR has to be equal to or higher than the peak demand flow rate gpm. The peak demand flow rate is the TOTAL water run through the softener or filter PER MINUTE.
Increasing water pressure increases the gpm flow rate. A good possibility is that the higher pressure has caused some tannin resin to be backwashed out of the tank OR... they don't have the correct DLFC (drain line flow control) in the unit and the time of backwash set correctly and the resin is not fully regenerated; hence it can't remove all of the tannin in the water now.
The use of potassium chloride instead of sodium chloride, regular softener salt, is another cause of insufficient regeneration. Softener and 'filter' resins are not in the potassium form, they are sodium form, so you must use a higher salt dose to use potassium chloride (salt substitute).
Since the filter worked before, a solution to your problem is to set the tank and pressure switch for 20/40 psi (19-18 psi air precharge when the tank is empty of water). That assumes there is a top basket in the 'filter' and you didn't backwash tannin resin out of the tank.
Another solution is a correctly sized softener/filter for your peak demand. You have what is called a top dressed softener; tannin specific (anion) resin on top of (cation) softener resin.