Why is water coming out out of the tub faucet and the shower head?

4 BRAND NEW tub and shower combos units installed later.........I FIXED IT!

I asked EVERYONE from certified master plumbers, 3 different fixture companies, to the know all, hear all, salesmen. NOTE, I USED 1/2" PEX. I was told the faucet was bad, so I changed it out. I was told the new one had a faulty diverter, so I changed it out and went with a new brand. I installed the new one and it continued to drop water from the shower head while filling the tub.

Next, I was told the water pressure was way too much and I needed a pressure regulator at the main entrance, so I installed that on my house also. Now, we had low(er) water pressure AND the original issue.

In addition, my wife was so disgusted she purchased a brand new shower unit and decided we would scrap the previous companies. By now I've about had it with everything, BUT, I'm not a plumber caller, I'd solve the issue.

Here we are 5 DAYS LATER, and I figured it out!!! With the NEW unit installed I had to go to the downspout, (which was 1/2" PEX) AND INSTALLED 1/2" BRASS TUBING. ISSUE RESOLVED..........THIS IS WHY!!!!

PEX is AWESOME AND I LOVE IT, BUT, when you use 1/2" PEX for the elbows, tees, and nipples it is restricted down to about 1/4". That was producing a head pressure back, or, pressure backup, which, forced water to backup into the shower head (looking for an exit). With brass, copper or galvenized (not recommended) the 1/2" turn are true 1/2" turns, unlike the PEX downsizing. GOOD LUCK and I hope this helped!!!