When the game is paused, any visible mobs that are burning will still be displayed as burning, even though they are not taking any damage.

Steps to Reproduce:

Step One: Load up a new singleplayer world with OP permissions (not sure if gamemode matters, I will test if it does)

Step Two: Spawn in a mob that burns in the sun in an area that allows the zombie to have direct access to the sunlight. (I chose zombies, but skeletons also work.) Make sure the zombie does not have armor on.

Step Three: Make sure you are looking at your mob while it is burning, and press the escape key. You should notice that the mob still has the burning effect while the menu is open.