As you can see in this image, when I lock the weight of this bone and add weight to a new bone after that, the lock didn't work properly anymore, the value of locking bone keep changing as if there's no lock at all:

Nate wrote

The weight percentages drawn in the weights view are rounded to the next highest number. For example, 11.00001% would get displayed as 12%. That way anything about 0% will show as 1%.

What is the full weight percentage for bone6 before and after your change? If it's just a tiny bit over 11%, like 11.00001%, then it will show as 12%. When the weights are adjusted, the math might work out to reduce it the tiniest amount, even though it's locked, resulting in 11%.

Actually the amount can be bigger as long as I drag the weight change for the new bone:

changing from 11% to 7% is quite alot