What causes stringy winter squash?
Well it's possible they are over ripe. Acorn squash should be dark green or mostly so when it's ready to eat. It turns orange when it's ripe, but just like many circubits and cucirbits (squashes and melons(including cucumber)) you don't want to eat them when they are ripe, you eat them before they start to ripen.

How you store them makes a difference in the long term but not so much in the short term. Last year we had a batch of acorn squash that was thin and stringy and not tasty just like you described. I think they didn't get enough water at the farm that grew the Walmart acorn squashes.

On the other hand, I have held on to acorn squashes for 2 years in the cool, and dark and they were still quite tasty.

Just try again, but try a different source for them for now.