The fruit looks as good as it has been.. Here it is from further away.

FYI, it's in a 5 gallon pot, north facing with light from the west, full sun, trellised against a banister. NYC.

By comparison, here is my wild cherry growing upside down, right next to it, and looking fine other than a few leaves looking overwatered.

Why would this happen to one and not the other?

Have you checked the soil to see how wet it is? It's possible it got less water than you assume it did. The foliage can block a good bit of rain from reaching the soil...

You are right. :)

I watered it in the evening, and by late night, it was looking good again. It gets sun all day long, but is slightly filtered by a tree above after a certain hour, and I guess those leaves plus the leaves on the plant blocked it from the rain. I am cautious about BER and overwatering, bc of so many photos of it online. So far, so good. Phew.