When you recast Sigils of Hope if you have Sign of the Guardian (increased block chance for each active sigil) then your block chance will drop by 3% for each active sigil when you recast it, so in my case I lose 15% block chance.
The drop lasts for a fraction of a second, but it shouldn’t. For argument’s sake it should at most drop by 3% for ONE sigil that gets replaced.
That is most likely what is happening. You reached the limit, cast another, the game unsummons one of the sigils to summon the new one. So for a fraction of a second you only get the bonus for your max-1, then quickly back to max.
EDIT: If that isn’t what is happening (and be aware of multipliers to your block chance elsewhere), then file a bug report, not a feedback topic (you can change the category by editing the first post).