I think Allen's nailed it. They over-acid-washed the finish and burned off too much plaster, and your pebbles are falling out. I suppose that might stop at some point, but then how many pebbles will be left? Or it might not. As your pool ages, I'd expect this to continue. That one pic of the grey hole, is that the exception or the norm? Just a few of those, or all over the pool?
It's disappointing to hear the PebbleTec guy claim this is within spec. I agree with Allen, let's see the spec in writing. There's no proof of the following, rather an instinct: you're not PebbleTec's customer. Not really. The installer is. The relationship for the PebbleTec rep to protect is the installer's, not yours. You're not going to be buying anymore PebbleTec from PebbleTec, but the installer is, and there is plenty of competition for those dollars, so the game is rigged against you.
This is a tough one, because my first advice in these situations is to get the PebbleTec guy to take a look! So this is what I would do next, or at least try. Get another one. Push back and don't accept that review. This works when I call a company for help and don't get the help I need. I hang up and call again and try to get someone else. If this guy is the regional rep, and the only one available, then I'd go over his head. The ol' "Let me speak to your manager!" routine. You need to make clear to PebbleTec that their first assessment is unacceptable and that you want another. If they refuse, then you'll need some stronger juju. And that might mean a third-party assessment, possibly some legal mumbo-jumbo. Let's cross that bridge only when we need to.
Next step is to try and get someone else from PebbleTec to the pool, for a second round.
And for what it's worth, from an admitted non-expert: that is not an acceptable pebble finish. Not even close. What
@Dtkokay said. the pebbles should be just slightly protruding from the surface of the plaster, and smooth to the touch. Yours would take off skin. That rep is lying to you (IMO), or didn't inspect it well enough, or both.
One trick, if you can get another rep to take a look, is to be armed with a PebbleTec sample when he comes. You can maybe borrow one from a local supplier for the next meeting. Ask the next rep if the sample matches your pool! Ask him if he'd like
his pool to look like this. This is right off the PebbleTec website, you can zoom in when it loads: