I have an Iron farm (This is the design: https://youtu.be/jY02cXSuN7M) 

UPDATE: What seems to be happening is that sometimes when the zombie is going up in the rails it burns and take damage for a fraction of a second. You can see it on this video: https://youtu.be/YXKxrHvQpeM. This video is from minecraft 1.17.1

UPDATE 2: When moving the slab ceiling one block up the zombie seems to no longer burn and take damage.