I'm playing as China doing the full Chiang Kai-Shek Central Club Clique route, and Subhas Chandra Bose is on India.
They triggered a communist revolution in Afghanistan and puppeted the country in 1958, which then gave me the option to back a rebellion under the "Support Afghan Exiles" decision.
When I click this button, about a day or a few hours later, I suddenly turn Trotskyist, join the Indian faction, and become a puppet of India, which completely locks the focus tree (semi fixed but still mostly broken even when trying to fix this with console commands and cheat mods).
I can confirm it is this decision that causes the bug, I reloaded a save three times and it was only triggered when I clicked the decision, and the bug was prevented by not clicking it and continuing the game as normal.
A bit annoying, but the bug is easily avoidable. Unless of course it isn't a bug and the Indians have 10 billion schizo sleeper agents in China dedicated to the sole purpose of preventing an intervention in Afghanistan lol