A week later, I've found the issue with my Mac Studio not going to sleep. I managed to narrow the problem down to the Studio Display. The Mac Studio would go to sleep with no display connected or the Thunderbolt Display. It turned out to by the setting "Listen for "Hey Siri" on Studio Display" with this enabled, it stopped my Mac Studio from sleeping.
Luckily Siri is something I rarely used, so I've just disabled it. Just posting in case anyone else runs into this issue.
Luckily Siri is something I rarely used, so I've just disabled it. Just posting in case anyone else runs into this issue.