Bit pity and odd the previous owner got a kind of rust problem with his frame. The Miyata 7 step finishing is part of the reason why I am so lucky with sourcing mint Miyatas all the time.
I do have a (yes, mint) Pro Am too. The one year earlier model, from 1984. Mine is equipped with 600AX/New600. You probably know the Pro Racer was offered as frame only as semi-pro offering. It resorted under the FullPro frame, that was available as full bike also.
Btw in your signature I see you own a Rafty Funnel. Dito. It was actually my main rider for on road, when I wasn't into roadbikes yet.
Well, the corrosion problem does not seem to be completely uncommon. The origin is the (salted) sweet that falls on the top tube. The soldered braces for the brake cable on top of this tube is one of the weakest spots in the lacquer and corrosion will start there first. This bike bike even starts to show the first signs. I am now trying to collect the full Dura-ace AX group after I will have the frame resprayed.
The Rafty I bought some time ago. The previous owner did not know what bike it was as it was repainted matt black. The Spinergy Roks wheels were broke as well (which not?). I am now sourcing for time correct parts for it. The company that produced the bike does not excist as it did then, but the are still around and turn out to be almost around the corner from where I live! I spoke to them some days ago as they still make bicycle parts and they have agreed to respray the frame and get the original decals for it!