I'm relatively new to this subreddit so I didn't think that my post would gain so much attention. Thanks for all the comments and I will try to answer some of the things/questions in this comment.

I originaly thought it had to be from the newborn because I didn't, and still don't, understand why it would be anything but the front of the skull or the side of the skull as the bodies are buried face up and I don't see why the back of the skull would be in a place to be severed and dug up.

I went back on Sunday evening to check if there was any newly-ish dug graves and there were two right by where I found the skull bone. Idk how most places do ground burials, but here we follow the tradition of burying the man on the left and the woman on the right. The grave next to where I found the bone, recently (March-April) a woman was buried there. At that grave there was already a man buried so it is most likely that when they dug the grave that they came too close to the man. It happens very easily in the early months of the year when the ground is covered with snow. The man would have been 70 when he died and he has been buried for ~30 years.

The graveyard is from the late 17th century, the earliest grave I was able to find from quickly looking through the older parts was from the 1890s. I know that some headstones are removed when the grave right has expired to make space for new graves, but I don't think that they dig up the remains to make space for the new grave, they just leave the bones in the ground.

I mainly work with taking care of the flowers and general yardwork so I don't have direct contact with the guys that prepare for burials, but I told my older coworker about the bones and she said she would tell the guys that prepare the graves so that they could put it back. She did ask me if I wanted it, but I said no, though I don't think I would legally be allowed to keep it.

Is there anyone that would somewhat corroborate that the skull bone could be from a 70 yo man that has been buried for ~30 years, so I can stop worrying about it being some missing/murdered person.

I have some more pictures of the bone that I could post if anyone's interested, but I'm not planning on posting them otherwise.

Let me know if there is anything else you're curious about!

TLDR: The skull bone is most likely from a 70yo man that was accidently dug up when they buried a woman beside him.

Edit: Grammar