Just wanted to update this, in case anyone is considering buying these tights.

The bunching-behind-the-knee thing turned out not to be a problem.

I've worn the tights twice recently. First on a rainy day when temps were in the low 60s. It was drizzle/light rain, not a heavy storm. This was a 30-mile ride, with a long time spent at the rest stop and then hanging out afterwards talking to people. The tights were comfortable, and they dried quickly afterwards.

Last weekend I wore them for a 46-miler with temps in the high 50s, maybe low 60s by the end. It was cloudy with some peeks of sun at the start, and I was generally okay but for a couple of minutes I wondered if I was overdressed. Then the clouds got thicker and light rain started, and these conditions persisted for the rest of the ride. I was glad I had the tights on.

This weekend I think I will be testing them in sunshine but with cooler temps.

So far I'm pleased with them. Thanks again to everyone who provided advice back when I was deciding to buy them.