It’s by design. Almost all blankets have polyester or some synthetics. It’s basically like sleeping under plastic bottles. Excellent for heat retention. Absolutely shit for heat regulation.

Pure cotton is better, but not as ideal as wool. Your life will change under a wool blanket. Wool blankets are insanely expensive, but you can find utility wool blankets for pretty cheap (very scratchy), you just have to sandwich them between some thin layers so you don’t feel the scratchiness. You can layer them multiple times and it is very difficult to have too many as they breathe very well.