Funky water pressure issues
If this is the gauge on the PRV output, you have a restriction on the intake side of the PRV or in the PRV itself. I suppose it may also be possible that your plumber installed a valve that's too small for your connected devices' flow rates.

As an aside, you also have a dimwit for a plumber if that's "a mystery" - or your plumber is lying to you, which comes somewhat lower than dimwit in my book.

You (or your new, hopefully competent and truthful plumber) will need to remove the PRV and see if there's something like an intake filter screen that is clogged with debris, pipe scale, etc. restricting flow into the valve, and if not, go looking for something stuck in the inner workings of the valve itself. You may wish to add a more easily serviced filter suited to the full street pressure on that intake side, as well.