Interesting Mikrotik GUI behavior

While bringing up a BGP session for a client I kept trying to add our side of a /126.  It kept reverting to the network address.  The video shows what happens when I tried to add ::12/126 to the IPV6 addresses.

After some second-guessing and then some Facebook chatting I decided to do a terminal /ipv6 address print.  Sure enough the proper IP shows up.  Must be a GUI bug.