I just recently took over a team in a Roster Only league in which I drafted a Kicker.

While on my cell phone, I added him to my depth chart by going into Details in his pop up card. From there I selected "K", which is the only option for a Kicker position. But in the depth chart, there's two positions: Kickoffs and Field Goal Kicker. When I went to look at the depth chart, I noticed he still wasn't listed in either of those. So while there, I clicked on "Add player..." in that position box (again, this is while on my cell phone). I then added my new Kicker to both positions.

Now when I click to view that Kickers pop up window, under Details, it shows him assigned to 3 positions. K, Kickoffs and Field Goal Kicker.

Is there a purpose to this hidden K position? (hidden because it doesn't show up in the depth chart) Is this a bug? Is this an overlooked/unfinished detail?

I'm surprised you're the first to report this. You're right, it's a bug, the hidden 'K' position does absolutely nothing.