Tire slipping on rim?

I'm dealing with what seems to be a rare problem, and hoping for some ideas here. I recently mounted a GravelKing SS+ 38 mm tire on a HED Emporia aluminum rim, with a 25 mm ID.  The previous tire, original to the bike, was a GK SK 38 that did 1500 miles with no problems.
I got the tire mounted up, tubeless, with no problems.  I was careful to have the stem lined up with the logo on the tire sidewall.  After my first 20 mile ride I noticed that the stem was rotated about 30 degrees away from the logo.  It also lost a bit of air.  I aired it up and rode 20 miles again a few days later, and after that ride the stem was exactly opposite the logo, and it lost some air again.

So, I took it all apart, took off the rim tape and cleaned everything with alcohol.  I applied a new layer of rim tape, mounted it up, and added sealant thru the stem.  It's no longer losing air, but the rim continues to slip inside the tire.  I've contacted both HED and Panaracer  but haven't heard back.  I'm nervous about riding this too far from home, or at high speed, expecting at some point this slipping could lead to ejecting the tire. 

I'm considering adding a few drops of tubular cement to the rim when I do the initial mount up, and see if that could prevent the slippage.  Any other ideas? Thanks.

Could be a couple of factors at work here.  Could be too low PSI.  Could be just a bad rim/tire combo.  Could be a bad bead on the tire.

It's not an unheard of issue and there are several dynamics that could be the culprit.  Seems to be rather common with Panaracer (google search funds lots of people making similar claims about this tire,)

Did you use an air compressor to install the tire or a hand pump? You can usually hear a pinging or popping noise when the bead seats. Inflate the tire to a psi higher than riding pressure when seating the bead.