Bug, UI or feature - tech tree edition

[Laiders]

Good evening ladies and gentlemen!

It's time to play everyone's favorite game show. That's right it's BUG, UI OR FEATURE...

Okay seriously I have a question and I'll link a screenshot below to illustrate. My MC currently say in the discovery subtask breakdown that 'a discovery is not currently possible'. I noticed a turn or two ago and switched most BP (well all but inertia etc.) to research instead. However, upon reviewing the tech tree to pick my next EC council research to ensure I advance the tech tree more, I noticed Heavy Combat Armour is not highlighted in yellow. This means it has not been discovered. What gives?

It's not as if it just became available because I have researched most other techs in the field and have a couple more only discovered. I can't think of another hidden precursor that might be required and hidden precursors would be bad anyway.

So is it a bug (HCA cannot be researched at present), UI wigging out/bug (can be discovered and researched), obtuse UI (cannot be discovered but is not communicating why properly) or feature or what?

Kinda significant because I would really like HCA and I would rather not lock myself out of discovering it due to some UI bug.