The pressure could be a problem. I had a PEX joint start to leak
about a year ago, and the plumber (who didn't have a pressure gauge)
turned down the pressure regulator at the main shutoff (it was wide
open). Perhaps it needs to be turned down even further.

Just don't call the plumber who adjusted the pressure regulator without
having a pressure gauge. Amazing.

Pressure regulators are generally also backflow preventors; they stop all
water from flowing back past it.
When the water heater comes on it will cause the water to expand, which will
increase the pressure in the system.
It can get high enough to cause the T&P valve to go. That is what everyone
is talking about.
An expansion tank will absorb the extra volume and stop the pressure from
going up.

I can only go by my own experience. I installed a pressure regulator to get
my pressure down from 90psi to 55psi.
I then let the water heater cool off, and turned it up high. I figured that
was the worst case senerio. My pressure only went from 55 to 85. I tried it
again and concluded I didn't need an expansion tank. I don't know why other
people do.
BTW, my pressure regulator will allow backflow, but that feature clogs up
and can't be counted on.