If I recall, the mindset changed around early season two. Before then we were foolish kids who thought it’d be funny to, at some point, end Paintbrush on a /joke/ of them walking into one gendered restroom, then out into the other, and out into the other, on loop- and that’s the end of them. At no point do I think we all collective had a single binary gender in mind, so it went from “heck if we know, let’s joke about it” in season one, to “heck if we know, let’s just not bring it up much anymore” in early season two, to “ooooh, there’s a word for that and it’s a rad, valid identity” I’d guess around 2014-2015. The fun thing there is that a lot of it wasn’t discussed. Many of us became educated on the subject, and I and a few others started pushing hard on “oh golly I know what we gotta do now.” It became important to us that we do justice to both the character we love and the many people who may have been hurt by our earlier poorer attempts at comedy and characterization at the proper point. We also knew that we really wanted to do a LB time travel plot since before season two started, and Brian pushed hard very early on for using that story to help LB understand PB- so it made sense to us to use that opportunity to give the audience understanding, as well, at that point. It additionally coincided with the point at which we were planning for PB to be eliminated, both fortunately and unfortunately for different reasons.
And I hope we served a favorite character of ours justice, per your judgement.