I made uncooked freezer jam from blackberries in my yard. I put the prepared jam in jars, placed lids but not too tight with 1/2 headspace. After letting them sit on the counter for 24 hrs ( for the pectin to set) the jam was seeping out of the lids and when I unscrewed the lid the jam almost popped out like an oozing volcano. No odor and I foolishly tasted it. It's been about 20 hrs now and I'm ok. I've made a lot of freezer jam and this has never occurred. To clarify, I used 6 cups crushed berries, one lemon, 4 c sugar, one cup corn syrup and 1 3/4 pack pectin. Then at the end I added an additional 1/2 lemon because I was worried it was not tart enough. ( I usually make my freezer jam with less sugar, less corn syrup and a little more pectin with no problem but I have never added lemon last).
Wow! That is a serious and impressive mess! Well, one good thing about jams and jellies, they are high in acid (low pH), so snagging that little taste wasn’t dangerous.
I have not seen this before and I am very interested in what happened. The picture suggests that your pectin gelation kept expanding after you put it in the containers. How did you mix the powdered pectin with your recipe? Did you mix it with water and boil it first (there was no mention of water in your description)?
Here are some general guidelines for jam and jelly making, but I will keep thinking about this.
With any jam or jelly, pectin is the critical ingredient to get it to set. Pectin can be pretty finicking and requires the proper ratio of acid and sugar to set properly. Acid comes from the fruit itself and any extra acid you add (in this case lemon juice). Sugar comes from the fruit and from the sugar and corn syrup that you added. Here are the recommendations from the National Center for Home Food Preservation for using standard pectin: https://nchfp.uga.edu/how/can_07/uncooked_berry_jam_powder.html
They recommend 2 cups berries: 4 cups sugar: 1 pkg pectin (6 Tbsp): 1 cup water.
Following the recommendations on the pectin package should also result in a good quality product that sets properly. Adjusting these ratios often leads to a jam or jelly that won’t set or is too hard. Adding more acid can make the pectin unstable and it can “weep”. There are low-sugar and no-sugar pectins available and these would be recommended if you want to use less sugar than called for in the standard recipe.