You should probably keep in mind that "the same time" vs at different times is not necessarily going to be visible to a human. It doesn't take much to be 1/60th of a second offset, and that'd give dramatically different behaviour. This is why all items are released simultaneously using a ...

No. Since the items all arrive at the same time, I would expect them to pass through the splitters undisturbed. Instead the first copper and iron plate get ahead when passing through the first splitter.

This does not depend on the belts being in a circle.
The behavior is identical if the belt feeds into a chest after the fast splitter.

Groups of items break up when going though splitters positioned like this: =======[===< ===[===[===< ===[=======< Blueprint: 0eNrtml9vozgQwL+Ln2GFbcCQh/sipxUi4LbWEYOMqbZb5bufgWyTJdCM5yL1Iu1L28Twm/F47PnjvpN9M8jOKG3J7p2oqtU92f39Tnr1rMtm/M6+dZLsiLLyQAKiy8P4Sf7ojOz70JpS911rbLiXjSXHgChdyx9kR4/BTUjfNqUJu1...

The behavior of splitters is affected by a belt being built beside it. Building beside the splitter changes the order of items on lines and can create gaps. At 0:18 the effect is even more pronounced, mixing different items Blueprint: 0eNrtml9vozgQwL+Ln2GFbcCQh/sipxUi4LbWEYOMqbZb5bufgWyTJdCM5yL1Iu1L...

Here you go.

Items can still jump forward when entering a new belt segment. The item jumps forward on the belt highlighted with hazard concrete, just before the red splitter. item creeping forward.gif I found several topics were similar effects are reported. https://forums.factorio.com/viewtopic.php?f=7&t=57...