When you enter a Nether portal while holding left click with a shield in my primary/secondary hand, reaching the other side, regardless of exit dimension, will cause you to have the shield up effect despite not holding left click. It is a bug that allows you to use the shield without the restrictions that come with using it (such as reduced mobility), but it consumes the durability of the shield you are carrying like a regular shield. However, you can still attack with melee weapons while the shield mechanic is still active (although it doesn't work if your back is to the enemy), and it even blocks attacks despite being in the player's inventory or from a chest. You can also place blocks, change items from the item bar, and it still will block everithing, and the only way to deactivate the shield is by holding again left click in air to raise the shield manually, wich will deactivate the bug. I'm not sure what could be causing this, but it's definitely from the latest snapshot, as it's happened to me in every world I've played in this version, and I don't remember it happening to me in snapshots before this one. I have not been able to upload video, but I think it is easily replicable with the information previously given. I hope it's of some use.