Do you happen to know what cultivar it is?
Some cultivars have brown-ish or bronzy stage for the new growth which could be the case, though new growth appeared at this time of the year still have no chance to survive winter.
Eventhough, such immediate reaction to first cold snap (what, mid-20's for couple of days?) is kind of puzzling.
Leaves on ericaceous plants don't die overnight.

You got hit with a cold snap that damaged any non-hardened off growth.

Dax

The Owner told me that the current advice for planting Rododendrons, Camellias, and Japanese Stewartia trees is to mix shredded pine mulch into the top six inches of slightly sandy clay soil. Even though the Rodos, Cames, and J. Stewatias are acid loving plants like Azaleas, which need the wetter peat moss beds, they need a better draining acidic bed of pine mulch mixed into the virgin soil.

Wonder if such advice is also true with you pieris japonica?

What kind of soil is in your P. Japonica's bed?

When amending special areas make them big enough to provide a full root run for the type of plant. Do not plant in small pockets, patches or strips of amended soil with unamended soil close by. The surrounding, unamended soil may weep water into the amended zone during wet conditions and wick it away during dry.

Amending for permanent plantings (trees and shrubs) has the additional problem of the amendments decomposing before the life of the planting is over. Installing more suitable soil on top of the existing soil, as a mound, berm or raised bed avoids the textural problems of an existing soil that is too heavy (clay-like) or coarse and dry (sandy).

Where digging in peat to acidify a soil and cause trace elements to be released it may be more to the point to treat the soil chemically. This will not help with drainage problems or lack of aeration, of course.

Bark, any species, has been recommended for all ericaceous plants both for improving soil structure and for innoculating beneficial fungi against the phytopthera that causes root rots, but don't consider it a cure-all for poor drainage. Bark is a good soil component for ericaceous plants in containers but plan to replace or refresh the soil every few years since all organic components of the soil will break down. Almost all container soils will be based on peat but you should of course select one intended for ericaceous plants since non-ericaceous potting soils have added limestone to make the pH near neutral.

Rhododendrons have been observed growing natively on limestone formations in western China. It's expected the moist mat of moss, roots and organic debris on top of the limestone is heavily leached by copious summer rainfall.

Two days ago, I moved my dormant Japanese Stewartia to an area which is fairly sandy on top of a heavy clay hardpan base only about 18 inches down. The about 6 inch thick layer of builder's sand is on top of the clay, and there was about 5 inches of fairly good loamy soil on top of the sand. Since the top soil contained such a low percentage of clay, I simply removed the grass, and mixed pine bark mulch in the top 5 to 6 inches of soil. after transplanting the tree, high, in this new bed, I covered it again with pine bark mulch.
This bed I created is almost 6 feet by 6 feet in area. Hopefully the roots of that tree will finally get the chance to establish well.

Since the virgin soil of the area is slightly acidic rather than alkaline, I am hopeing that maintaining the acidi level needed, can be expected by properly bark mulching each spring, and if need be, also in the fall.

Needs to be postioned away from harsh winter winds.

Is your shrub provided with all these above listed elements in it's planting area? If so, consider the following:

Leaves which turn brown but do not fall usually indicate root stress from poorly draining soils; or if the stems also seem damaged, young growth that dies from temps which dropped below freezing before that vigorous new growth has had the chance to harden off.

Most everything you described suggests that the soil does drain well; until, you mentioned that the shrub is planted along a crude rock retaining wall. What happens to the runoff which comes off the top of the ledge on the other side of your retaining wall? Also how much water which filtrates down through the sandy loam soil, which rises up behind your retaining wall, flows through the crude rock retaining wall and onto your P J's planting bed?

Even if your soil drains well, an excessive duration of runoff or filtered through water would stress your shrub.