So I was playing a save with the TETRIX tech tree and I noticed that for some reason I was missing the KR-2042 part from kOS... after looking through the config files, it seems like "@TechRequired" property of the part was set to "Start" with a capitalized S, which apparently is not the same as "start" since after changing it the part showed back up
TLDR: BUG
KSP-Version: 1.12.5.3190
MOD-Version: TETRIX-2.26.1
Bug: Missing part (KR-2042 from kOS)
Fix: change "@TechRequired" in @PART[KR-2042] to "start" instead of "Start" inside the "TETRIXTechTree-ModParts.cfg" file