With only K2, default rocket fuel stack size is 40 items per stack. With SE+K2 it should 2 items per stack but it is not. Suggestion to correct below:

With the later integration mod of K2+SE, the rocket fuel stacking feature seems to be looking at K2 instead of SE post-process (which changes K2 default rocket fuel stack sizes). Could you change your code base to 'if SE is active, look at SE-post-process mod instead of K2 for understanding stack size of rocket fuel'? That may fix the issues. Thanks

can you give me a save file so I can see all the mods at play

Here you go. Unfortunately the update you just made did not fix it. The stacked size should be 2 (as the non-stacked size is 10), but instead it is 40 (20x difference) . Godspeed and thank you for your help and quick reply!

https://drive.google.com/file/d/1CcPXY2iaRTkEK1Bv5wAe_j8N4od2zm8U/view?usp=sharing