Sulfur processing research needs only vanilla red and green science. But, 4 of the prerequisite researches need science packs 15 and 28. (those 4 are Engine, Automation2, Fluid Handling, Oil Gathering)

15 requires sulfur and plastic to make, so I am unable to unlock sulfur and plastic processing without already being able to produce sulfur and plastic.
Similarly 28 requires uranium, but is also needed to unlock uranium mining (which also needs sulfur).

All mod options left at default so the recipes shouldn't be randomized.
Unless I miss something this is a deadlock.

In my game, many of the early researches need packs 1-4, 10, 15 and 28. Looking at Mikael Henrik's playthrough, the science packs are generally needed in order, so he doesn't have this problem. I have no other recipe-altering mods, just stuff like even distribution and textplates. What did I do wrong?

Am trying to debug. I think that this is caused by the fact that sulfur and plastic both have a direct prerequisite of "Oil Processing" which is a tech that is unlocked by pumpjacking oil, rather than doing research. The code in functions.lua:41 marks techs with a research_trigger as 'invalid' which means in prototypes/technology/preprocess.lua:66 it is marked as 'disabled' and therefore removed from consideration. So we end up with sulfur and plastic (and therefore science-pack-15) having no prerequisites when deciding what science can be added to research. Similar situation with the uranium products, as "Uranium Processing" tech is also triggered by mining rather than research.

Here are some relevant log lines:

prototypes/technology/preprocess.lua:32: Technology sulfur-processing has a prerequisite oil-processing.
prototypes/technology/preprocess.lua:32: Technology plastics has a prerequisite oil-processing.
.....
prototypes/technology/preprocess.lua:67: The prerequisite oil-processing of the technology sulfur-processing is disabled.
prototypes/technology/preprocess.lua:67: The prerequisite oil-processing of the technology plastics is disabled.
......
prototypes/technology/science-pack-prereqs.lua:13: The item sulfur in science pack 15 might be uncraftable
prototypes/technology/science-pack-prereqs.lua:13: The item plastic-bar in science pack 15 might be uncraftable
.....
prototypes/technology/crafting-hierarchy.lua:175: Recipe plastic-bar is uncraftable due to the lack of capable assembling machines.
prototypes/technology/crafting-hierarchy.lua:175: Recipe sulfur is uncraftable due to the lack of capable assembling machines.
[NOTE: this is repeated for almost all recipes made in chem plants or centrifuges]

This looks like a reasonable chain of events to cause this problem, however:
1. I have no idea how to fix it. I can only read lua, not write it.
2. Surely this would affect everyone - how come I am the first to notice? weird.

Yeah also just reached the recepie conflict kinda sadge hope this will be fixed soon even if it only is a side project for nihilistzsche