Reproduce

1. Change player's attribute via command

   /attribute @s minecraft:generic.max_health base set 1.0 

1. Die and respawn

   /kill
   

1. Check the attribute

   /attribute @s minecraft:generic.max_health base get 

The value is 20.0f as default rather than 1.0f which is expected.

This bug is related to MC-16773 to some extent, when Adams implied this may change after "we have more control over how attribute modifiers are applied." But now we still don't get a method to keep a changed attribute value during respawning, though attribute command is provided.