Glad to hear that the OP got this sorted.

I had a similar situation after meticulously rebuilding the old Shimano 600 headset on a 1981 Bianchi just a few weeks ago. What was odd was that I adjusted the headset with a dummy stem in place to simulate the expansion of the quill in the steerer. I had it perfect and with the locknut good and tight, but after the first ride the headset had a loose adjustment! No, the locknut hadn't loosened at all, but apparently the bearings settled into their grooves a little better(?). Weird is all I can say, but I can say that I've seen these things happen, somehow.
Perhaps the cups or crown race finally settled more fully onto their press fit, even though the old bike was used and I hadn't disturbed the press fits at all during the rebuild process.

On the other hand, In 50 years I have come across a couple of headsets that were poorly machined such that one pair of the properly-assembled cups and cones rubbed slightly because of zero clearance, even with the correct size balls in place.

Oh, and on my own PH501, those headset components were HEAVY in the extreme, which is odd in that the entire bike weighs only a bit over 22lbs iir.