Shield deflects damage while not up after going through a nether portal.

I was entering a nether portal with a shield equipped in my off-hand.  When I entered the nether my shield appeared to be down and I could still run/sprint.  I needed to kill a piglin that was in my way but I realized that I was not taking damage but shield was losing durability.  I understand this bug can be reproduced while holding up a shield while entering the nether, but during this incident I was not interacting with the right mouse button at all. The only mod I was using was optifine and there were no data packs installed.