Wardens won't pathfind through lava even though they're immune to it

Summary:
Wardens won't pathfind through lava even though they're immune to it. This is inconsistent with other mobs that are immune to lava, which will attempt to walk through it to get to the player if there is no other option. Java report about exactly the same problem MC-249415.

Steps to reproduce:

1. Summon a Warden.

2. Create a lava obstacle for him.

3. Force the Warden to go through the lava.

Observed results:
Warden cannot walk on lava (despite being immune to lava).

Expected results:
Warden must walk and pathfind through lava.