Esteemed colleagues,
do you use trivets? I use a small (one of the smallest) Kuhn Rikon and 0.25l jars that I place on a shallow trivet that was included with my pressure cooker. I tried several variations of different garlic varieties, oils and herbs, and each and every time my results were very consistent.
What you describe might be happening due to excessive heat near the point of contact of your jars with the bottom of the cooker. Try trivets!
I should also note that I use gas stove, I slowly pressurise my cooker at low heat (no point rushing it) and then leave it on a very, very low heat ”” it's more than enough to keep the pressure. That way the bottoms of my jars aren't exposed to excessive heat.