Found Nether Stone below my highest realm depth

Posting this, though it might also be me either misunderstanding or it having been changed without me knowing.

Basically, I’m at realm depth 180, but could have chosen 181 as my highest realm depth, and I found a nether stone in a chest (low rarity with 20, but still), and as far as I’m aware they should only appear on the highest realm depth.

So either this is a bug or I am misunderstanding what the highest realm depth is. Since I haven’t yet finished 181, is 180 my highest? Or were nether stones updated without my knowledge?