In prototypes/compat-updates.lua (1.1.5, lines 198-205) the K2 matter helper is called with unlocked_by = "molten-plastic-processing-advanced-kr" before that technology is created at lines 207-274, so the helper finds no technology and adds no unlock. kr-matter-to-molten-plastic is still listed in the technology's own effects, but the opposite conversion, kr-molten-plastic-to-matter, ends up unlocked by nothing. Creating the technology before the helper call (or adding the unlock afterwards) fixes it.

Seen with Krastorio 2 Spaced Out 2.0.17 on Factorio 2.1.20.