I flew into a nether portal on the nether side and died on impact with the blocks directly behind it. When I got back to the nether side portal my items were gone and checking the overworld side of the portal, they weren't there either. I checked the entire area below where my portal was on both sides and couldn't find my items, not even the Netherite gear that would've survived falling in the lava that was there on the nether side. Yet when I recreated the event in a copy of my world, exactly how it had happened the first time, the items I'd given myself appeared on the overworld side of the portal and even after this, I couldn't find a single portal that had generated elsewhere in the overworld using the spectator mode experiment I'd activated for the copy.
My items were completely gone instantly in survival