Just because the pressure at the main measures 70 PSI doesn't tell us anything about the condition of the piping in the house. How old is the house? And what kind of piping do you have? If it's old, galvanized piping, it is likely to be extremely corroded on the interior, and this would be the likeliest reason for pressure fall-off. It's like a human with clogged arteries.

kudzu9, thank you for your reply.

The type and condition of the plumbing in the house are irrelevant. The regulated pressure is being measured immediately downstream of the pressure reducing valve, indicating that the pressure drop is happening in said PRV.

FWIW, the plumbing is 3 year old PEX.

Watts is a good, quality brand, and besides, regulators aren't rocket science. Something's not right with your regulator or its setting. Looking at the specs for a 3/4" regulator, your 25 psi pressure drop should occur near a 30 gpm flow rate, that's like 10X what a shower would flow! Could the regulator actually be set at 25 psi and be leaking up to 50 when static? Could the regulator seat be damaged and need replacing? How about trying this: turn the shower on and bring the regulator up to 50 psi outlet pressure. Then shut the shower off and see what happens. FWIW, my pressure reducing regulator is set to 70 psi output and we have a nominal 70psi inlet. (I added the regulator because I caught my town water pressure spiking to 130 psi once!)

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/es-lf25aub-prod2.pdf

Mike- I agree that you shouldn't have a problem with the type and she plumbing you have. The way you now describe the problem clarifies where the problem is....