In the snapshot 20w22a the dragon death animation was broken (MC-186274)
It was then "fixed" in 1.16 pre-release 1 but not completely as it now had various rendering artifacts. Then the animation was broken even more in snapshot 20w45a. On both occasions noone reported it and it's been broken ever since. 

According to one shader dev I asked, about this they said "the dragon death animation renders to depthtex1 for some reason, so the game incorrectly assumes it's solid, instead of translucent" (depthtex1 is an iris/optifine shader pipeline thing so not relevant to vanilla, but I was told the "the game incorrectly assumes it to be solid" part might be relevant to the vanilla bug)

Before 20w22a (no issues)

After 20w22a (broken)

After 20w45a (very broken)