If the top is growing (which it obviously is) your moisture levels in your basement are too high- in relation to the subfloor moisture. Did you do a moisture test prior to doing the work? Did the floor acclimate prior to install?

As far as the tread goes...

I believe the board behind the one with the bullnose is growing, this is what has pushed the miter apart. It's hard to tell without being there.

You need someone to do some moisture testing on your project if you want to get to the bottom of what is going on.

Alan

well, the pictures of the stairs are in my wine cellar. And it is certainly humid in there - that's the goal. As to whether there was a moisture test I cannot tell you - but I'll see my GC tomorrow and ask. He did say the subfloor (concrete) cured well over 30 days.

and by "growing" do you mean what I understand "crowning" to mean?

Take out the humidity and it will level off. You have a foam substance

under the very thin layer of wood, right? The foam grows when damp/wet.

This may sound redundant but the crowning you are experiencing is from moisture. The moisture content above the hardwood is higher than the moisture below the hardwood(i.e. the subfloor) The proper Relative Humidity(RH) for hardwood flooring is between 30-50%. Some say that 40-60% RH is OK. Anything higher your flooring has the risk of buckling. Even if the humidity doesn't seem that high in certain rooms- it may actually be. I would invest in a RH reader. They sell them on Amazon and we had purchased them for many clients.

Moisture Reader on Amazon

If you click on the link above- it is the moisture reader we have purchased. It reads the temperature and moisture content in percentage. It also has a gauge which will tell you when the RH is low, high or just right.

Cupping vs. Crowning

Was this a glue down, site finished floor? If, yes, the flooring was sanded to early. The moisture in the wood did not return to the original reading prior to installation. The floors were sanded, then drying was completed afterwards, leaving you with crowning. Repair will depend on the thickness of the veneer.

How was the flooring installed? Floating or glue down? If glue down, what glue was used? How long did this have to acclimate in the SPACE (with all humidity ranges set at the appropriate levels, etc) BEFORE it was installed? How much expansion gap was left at the walls? Can you pop off some trim to measure it? Photo of the measurement?

Wood happily expands in the presence of moisture. It is hydroscopic. That's what it does. That expansion/contraction is supposed to happen BEFORE the installation. That's why the acclimation in the CONDITIONED space must happen BEFORE the floor is installed.

When you pop off the trim, you can do a quick guestimate to figure out if enough expansion was left around ALL vertical obstacles (door jams, walls, corners, etc): the expansion gap is often the same as the THICKNESS of the floor.

That means: a floor that is 3/4" thick plank (19mm) should have (roughly) 3/4" expansion gap (19mm). A 1/2" plank should have 1/2" expansion gap. This is a rough estimate of what is needed. You get the idea. Go ahead and pop off some trim and measure the gap.

What is the humidity reading in the wine cellar and in the rest of the basement?

well, my GC and his flooring guy are supposedly meeting me at my house in the morning. I've sent the explanations above to my GC. Not sure what he will do. Part of the issue in the wine cellar is that the racking is installed above this floor and it's full of wine! (A picture of part of the space is below.) I guess I could let the humidity go down some (it was hanging out at 70% this summer - that's ideal for a wine cellar). Frankly, I wanted to put in a cork floor but my GC said the engineered wood would work in there.

If the humidity drops to closer to 50%, will the boards retreat to a proper shape or do I still need to have remedial work done? And I'm curious about what is causing the problem elsewhere in the basement, where it's not nearly so humid (probably well below 40% now.)

Oh, and when I get home I'll check the humidity levels at present and report back.

Is the issue only in the wine room and bedroom? Or is it consistent throughout basement?

First, your wine cellar is absolutely beautiful. Some can only dream of having a space like this in their home. Simply stunning.

How is the flooring cleaned? I'm asking because maybe that is one of the culprits causing this issue in the other areas of the basement. Maybe there is too much moisture being applied to the floor. It's hard to definitely tell what is causing this issue without doing a proper walk through and looking at all check points.

If the humidity drops, there is a possibility the flooring will retreat to normal condition. But there is also a possibility that it won't.

SJ McCarthy makes a valid and strong point, definitely check if there was proper expansion space left. Good Point.