I don't see the logic of cooking a jam for nearly an hour and using pectine sugar. I would have used plain sugar in your recipe, that would make more sense when you say you reduced the mass by half; in that case, you have ended in an almost 1/1 ratio tomato mass/sugar.
I have no idea about the bitter taste, could be anything.
I make a lot of jam using pectine sugar, adding the sugar to a boiling fruit mass and counting from then, boil vividly for maximum 4 minutes; I usually go for 3-3,5 minutes. The jam sets perfectly when cold.
When making fruit jam with pectine sugar, I mostly use a ratio of 1/1 fruit/sugar or lower going to 10/8 fruit/sugar.
My best suggestion; (important; start by weighing your empty cooking pot!!)
- cook and reduce your concoction without sugar for maybe half an hour to develop the taste,
- let it cool overnight to let all tastes merge,
- weigh the cold mass (minus the weight of the cooking pot!!),
- bring to a boil while stirring continuously until it boils,
- add the same amount of pectine sugar (1/1 ratio) (or go to 10/8 ratio), count maximum 4 minutes from the moment the boiling resumes.
- fill your jars while very hot
Note on the jars; Just before cooking the jam, I wash the jars, put them still wet in my oven at 110°C (not the lids!). When the jam is done, the jars are very hot, sterilized and dry. I fill the jars immediately with boiling hot jam. The jars are too hot to burst from the hot jam. Fill the jars, put the lids on without using force and turn the jars upside down for the next 45 minutes; they will vacuum themselves.