I mean, I don't think it's an unspoken rule as much as it's a pattern you've noticed. And they didn't "go out of their way", it's just a movie script.
The story they wanted to tell for Thor 4 was
meant to be relatively shortly after the events of Endgame. L&T could have easily been the first (or second or third) movie released post-endgame, but there's plenty of logistical factors for when these are filmed/released that have nothing to do with timeline placement. Location stuff, Hemsworth's contract, bringing Portman back in etc.
Technically, all 4 phases have at least one movie that jumps back a little bit in the timeline.
Rman:
There's no way to determine exactly how Groot ages. I just don't think his growth can be used to determine anything. Clearly the Holiday Special shows some time has passed since Love & Thunder but we can't say for certain how much.
Right, he has alien physiology that we know doesn't behave anything like a human's.