Well! That is interesting. I saw something similar on a couple Jarrahdales that weren't quite mature and got nipped by the frost. Yours doesn't look exactly the same as mine did, though. I sure hope someone can chime in with some ideas.

I don't want to gross you out too much, but I had a problem with squash vine borers boring into the squash fruits this year.

Not that that's what you have! But that's the first thing I thought of.

Except, my bored fruits soon rotted and got squishy, so it was easy to tell which ones were compost fodder and which ones were good.

I baked my Waltham butternut squash in the oven and then pureed it for freezing. I found tiny round creamy little white dots on the seed cavity of one of them. Did you see any of those?
There wasn't any kind of disease anywhere on the squash, so I went ahead and used it. Then I got to wondering if it was some sort of worm or something that turns into a white little ball when heated? Yuk. Oh well. I don't know which quart it is in the freezer, so I guess we'll just eat it!