Parthenon courgette

I'm growing one of these this year for the first time, in my tunnel. I was kind of expecting that because they don't need pollination they wouldn't do the usual courgette thing of fruit appearing then rotting and falling off. Mine showed its first fruit a while back now, and it was puzzling me because they were just sitting there not growing. Today when I looked they seem to be withering, so I suspect they're about to rot. There's some new fruit showing higher up on the plant, and these seem to be growing normally.

So is it normal for Parthenon to do the rotting fruit thing, even though they don't need pollination? If not, what else could be causing it? They're being watered via a sunken plastic bottle alongside the plant, so it shouldn't be water on top that's causing it (and certainly not rain, in a tunnel).

The funny thing is the plant itself has actually looked stronger (and the stronger new fruit appeared) since the weather turned colder. My outside squash and courgettes are looking rather miserable at the moment, apart from the ones on the compost bins, which are looking very happy, but the Parthenon seems happier since the temps have dropped a bit. It was getting ridiculously hot in the tunnel when we had that very hot weather, so maybe that's what stressed the plant and stopped the fruit from growing.

I  grew these last year and found that any damage from slugs etc when fruit was tiny, led to rotting.

Once the fruit was past a couple of inches, they grew fast, but it was warmer last year.