I don't think it's your game. Same thing happened on all mine too. Kinda rough actually because now I don't know if my "old" people died or if a disease struck my village and with 90, I can't remember whose who and it's near impossible to find the sick ones unless you scna the details of all 90 villagers.

Just seems to be the way it is.

It'd be nice if in an update the villagers start replacing the earliest tombstones with the new ones. I could care less who dies a week ago now!