Not yet clear why, whether that is bad manufacture, most likely, or due to the way I wait for a full load of washing before doing that so that some sit for a while before being washed. Unlikely to be the last because the previous ones I used didn't do that and were treated the same way.
Since they are only small holes, not big thin patches, I have decided to try darning them.
Easy to find the needles and eggs/mushroom things to do it and my mum taught me how to do it when I was a little kid. But its amazingly hard to find some decent wool to use to do the darning with even with the word wool in the search term on all of ebay, amazon and aliexpress. Most is cotton or synthetic even when it has wool in the title.
Thought occurred to me that the obvious source of wool to use is one of the sox.
Anyone know of any good reason why that wouldn't work ?