I had altered my own minecraft:generic.attack_damage base value to 5.0 before fighting the Ender Dragon. After defeating the dragon and returning through the portal to the overworld, I skipped the credits. Shortly thereafter I went to kill a chicken and discovered my unarmed damage was back to default. I confirmed with "/attribute @s minecraft:generic.attack_damage get" that it was back to 1.0. 

Additional testing shows it retains the value when I enter The End, but resets it when I travel back through the return portal a second time. Altered attributes also reset if you die and respawn, which makes sense, but it doesn't make sense to do it when returning from The End. I also tested it with a Respawn Anchor set in the Nether, and again attributes get reset when returning from The End. (At least it isn't using up a charge any more!)

Steps to Reproduce:
1. Use "/attribute" command for example like this: "/attribute @p minecraft:generic.max_health base set 40"
2. Return back from End after defeating Ender Dragon
3. All attributes reset back to original. Something interesting about it is that how many HP you had is still visually visible, but it is actually reset