Notes:

This is probably the most relevant discussion about rulings the series has covered so far. Anywho it's confusing wall of text time.

The De-Fusion/De-Syncro interactions and remembering what materials are, are a small part of an interesting set of rules concerning what is and isn't considered a Fusion/Synchro Material for a monster when certain things happen. For example, you can use De-Synchro to send Stardust Dragon back to the Extra Deck, but if you used its effect to tribute itself and Special Summon itself back in the End Phase, you cannot Special Summon the materials for Stardust Dragon because the materials are no longer considered the Synchro Materials for that Stardust Dragon. The Perfect Rulebook keeps it simple: if a Fusion/Synchro monster is destroyed or leaves the field (except if it only temporarily gets banished), the Fusion/Synchro Materials that were used for its summon are no longer treated as its Fusion/Synchro Materials.

Then there are the really counterintuitive rulings about what you can and can't Special Summon off of De-Fusion. Starving Venom Fusion Dragon is a perfect example. You can activate De-Fusion on Starving Venom and send it back to the Extra Deck, but you can't summon the materials back because the materials in the graveyard aren't "DARK monsters on the field". Another example is how you can't use De-Fusion to Special Summon the 3 Proto Cyber Dragons you used for the Fusion Summon of a Cyber End Dragon because they are not "Cyber Dragon" in the graveyard. It seems that the materials in the graveyard have to still be applicable as Fusion Materials as listed on the card text of that Fusion Monster. Very counterintuitive and silly sounding, but it is what it is.

Link to the Imgur album. https://m.imgur.com/a/1rLokdy