Not sure if that counts as a bug, but in Polish common names have both words written in small letters. Example:
Cepaea nemoralis would be written wstężyk gajowy, not Wstężyk Gajowy - as it currently displays and is orthographic error.
Something to consider. :)
Sadly for now it’s impossible to make first word appear with small letter, so consider it’s as a title. You can add names the way you want though (at the botton of taxon page they’re shown he way they were added), maybe one day it will be changed.
Because of the massive complexity of dealing with all the rules, exceptions etc across hundreds of languages, the site has decided upon a standard display format of capital case for all common names.
So this is not a bug, but rather intentional behaviour.
Many languages have no rule or agreement on how they are formatted. Try getting consensus if English bird names are capitalized or not.
Managing it through the entry was dropped as it was impossible to maintain or manage as people would simply not know or worse knowingly disregard the guideline because they disagreed with it.
This along with the constant edit wars changing priority of names etc was the reason editing of names was restricted to curators and the standard display format adopted.