In backwash mode, I would expect about 5 PSI, not 15 PSI. But I don't know what you have on your backwash plumbing either.

But in normal mode, you get 23 PSI, I would expect 15 PSI and without the heater you get 20 PSI and I get 11 PSI.

There seems to be a 8-10 PSI delta.

Does your pressure gauge go to zero when the pump shuts off?

If so, this tells me that perhaps there might be an issue with the backwash valve.