I was fighting the ender dragon for the second time in my 1.15.2 world when i had to go eat lunch. I left the world, went back in, and realized the dragon was still on the bedrock podium in the middle of the end after I attacked it a decent amount. I decided to do damage, leave the world, go back in, and attack it again, and sure enough, it let me kill it without leaving the podium! (Note: I had no mods, potion effects, enchantments on my diamond sword, or was in creative).

How to reproduce:

1. Respawn the ender dragon.
2. Destroy the end crystals
3. Wait for the ender dragon to go on the bedrock podium at 0,0
4. Attack about 1/8 of the dragons health.
5. Leave and re-enter the world.
6. Repeat steps 4 and 5 until the dragon is dead

Good thing you have to fight the first dragon legitimately!