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This didn't happen before NSB. I think it's clear evidence that the AI is performing naval invasions without really having naval supremacy. And that the AI can't defend against naval invasions at all.
This happened in my last Japan game, I'm pretty certain that I never lost control of the Belt and there wasn't a single UK division behind my lines so I also do believe the AI is ignoring naval rules.
Nevermind that I probably sank the entire Royal Navy by then too.
Disregard this entire thing, I managed to find a savegame right before it happened when I decided to double-check it and there were 6 divisions and 12 destroyers in that tiny island under Skane obscured in the picture and they had been routing supply back to land and then back down from Oslo.
That being said, you only need naval supremacy for a single tick in order to launch an invasion. What's more likely is that Japan usually has their fleets doing other stuff in other regions (supporting its own invasions) and left the way to the Home Islands open.