A shield remains in a blocking state after a player holds up a shield and travels through the Nether Portal. However, the shield animation itself is in its default lowered state and the player is able to walk around at a normal speed and freely attack and interact with blocks and items as if their shield was down. Essentially giving the player a wall of invincibility in front of them.
This can simply be recreated by holding a shield up while entering a Nether Portal, and can be seen in action if you are to be attacked by a hostile mob. Raising the shield and lowering it, or drawing and firing a bow will remove this effect.
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-86252, which is a report that has not been resolved yet.
If you want, you can upvote the parent ticket, and enable watching to receive any updates about it - if you believe you have any information that may be important for this issue, please leave a comment on the parent. Please also use the search function in the future to prevent duplicate reports.
Quick Links:
📓 Issue Guidelines – 🛠 Community Support – 📧 Customer Support – ✍️ Feedback and Suggestions – 📖 Game Wiki
💬 Mojira Subreddit – 💬 Mojira Discord