The $si is one of the oldest variables, being in this form from beginning. Not sure about others, but I use it in many API requests or sqlops queries. Moreover, the definition of an IPv6 address is without brackets. The brackets are in the grammar of an address or URI. The $si won't change, I can bet is going to break a lot of configs. I added $siz to master branch that should return the ipv6 address in between brackets.
I am closing this one being a config issue, not a code problem. If you still think there is not enough space in the static buffer, open a new issue pointing in the code where you think the problem is.