both chaos and sleep fall in same CC category what cannot be overlapped so when you get opponent is under chaos and that unit just happens to be target who izuna puts sleep so it overlaps that chaos changes to sleep debuff instead. (both chaos and sleep affect movement of unit making them ether miss turn or attack their own teammate so those 2 cannot coexist on same target)

(info here might not be 100% accurate)

Is it just those 2 that can't overlap?
because,I don't remember that ever happening with immobile or blindness and other controlling debuffs in the game
But If it IS working as intended then Izuna is going to screw me over every time... THAT DORK

blindness and immobile are is same category as well making them do similar thing but there is difference in actual effect as blindness is makes unit "blind" so it cannot see where attack or movement goes while other hand immobile stops all type of movement what unit can have. (as in blind person can move but it cannot see what he does and person who is immobilized person cannot do any type of movement. (So in game they both affect same thing but same time they are different so they can coexist) Sleep and chaos other hand are both affect in some way of Psychological dmg (forcing person to sleep or making them confused so they see teammates as enemy)

(still not guaranteed that info is 100% correct)

I have been testing these things against my other account.
I can confirm that you were right about the Chaos-Sleep thing but not the Immobile-Blindness.
I used Fu Yamanaka to chaos then fire main to sleep and then Lee to wake up and yes the sleep OVERWROTE the chaos and the target woke up,
I tried it the other way around with fire main first to sleep then Ino to chaos with her chase -also tried with Fu-, then used Tsunade to disperse "ONE" debuff off and the target recovered both of the debuffs.
However, it does NOT work that way with blind - immobile.
They do overlap.. the target had both of them and Tsunade healed but only removed 1, they don't overwrite each other.
I guess I should have tested this myself before making the post , but I have played the game for a long time I thought I knew it all
[ Funny how I used Ino with fire main a million times before in the Arena and Zenith and never actually realized she "Replaces" the Sleep not overlapping it]
Thanks for enlightening me bro, u were a great help.
I learned something today : fuk Izuna.

np :P ye too many variables and as retired player, so didnt remember how immobile and sleep stack went but as in old days used sleep, chaos cancer teams (1.0 -3.0 meta for me. ) so that part stayed in some what in my mind :)

Blindness can be applied on top of other hard CC's like Chaos and Immobile.

Sleep/Chaos and Immobile/Chaos are the only pairs of debuffs which overwrite each other.

If you r suffering from Chaos and you got sleep'd, when you clear the Sleep, the Chaos disappear!

here's the link to my thread explanation about debuff cleanse.