Server Resetting Issue

Why does my server resets even when I do save-all before leaving? It happened 4 times already. I'd like to know information if you could give me some. Thank you

Fixed it, I just turned off "Optimize". Tho it's pretty weird because that world was generated by simply Minecraft, no other softwares.

Yeah, but since it didnt, I tried doing it anyways manually. But it's alright now, thanks anyways.