An update: For some reason the issue was solved on Windows simply by running the “powercfg/a” command in the command prompt to check available sleep states. I no longer think the issue is with my motherboard, as it also slept fine on Windows before I’d started Linux. It’s possible the windows issue was just a fluke. It now sleeps properly.
Ubuntu still will not sleep properly however. I could have sworn I previously found other people with the same issue, but I cannot find them anymore.
Either way, the issue appears to be with Linux unlike previously thought. Possibly an incompatibility with my motherboard.