Always effective shield

When I try to enter a nether portal with my shield holding,I found it always be effective in nether.If I don't use it again,it's always keep guarding. 

[my english is not good but I hope you can understand this.]

What I expected to happen was...:
When I hold my shield and enter the nether portal,it should be invalid.

What actually happened was...:
In fact,it is keeping guarding and I don't need to hold down the right button.Even I can attack.

Steps to Reproduce:
1. Hold a shield
2. Enter a portal