Wardens have been made immune to lava however they can't swim in it properly. This isn't in parity with Java.
Steps to reproduce:
Place lava on the ground.
Place a warden in the lava.
Have a mob or player (must be in survival) make enough vibrations to anger the warden.
Observe the warden not move in the lava after being angered.
Thank You for your report!
Currently we are tracking this issue in MCPE-153582.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.