Cracked bottoms of jars

LC question for y'all.Are you tightening your jar lids before the PC or leaving them loose? I've been tightening mine but I keep having the bottoms of my jars crack and break. Its getting annoying af. Since they have syringe filters I'm wondering if it even matters whether they're tight or not? Like the air can still escape through the filter right? I'm thinking maybe it's a heat thing because it's literally always the bottoms that break. I do have my canning rack elevated with jar rings underneath so idk if that's part of the problem or what. Has anyone else dealt with this? What am I doing wrong?