Had a look a bit deeper into it and I think @kuttermax and @Tanner1280 have it right. I know it’s not something anyone wants to hear, but your FTP is set too high.
On 2 July you did a ride with a TT effort in the middle of it. Your highest 20min power on that ride doing the effort was 316, which suggests an FTP of 300 or possibly a little lower. There may be some variation. But I’m fairly confident in saying it isn’t 329.
So the first 3 hours of the ride at NP 250 were more like IF .83 than .77 - which makes quite a big difference.
I think the other thing to note from your recent rides is that you haven’t done much structured training recently. Which is fine, it’s summer and nothing wrong with getting out on your bike and just riding. But unless you put in some proper long intervals either outside or on the turbo, your muscle endurance is likely to come up short when the intensity goes up.