• Member Settings → Profile Page Options
• Description length limit is off by 4 characters, i.e. my description is exactly at the limit, I can type four more characters, but after saving and returning to edit it those final four characters are gone (and I can type them again)
• Firefox 45.0.2-1 (Arch)
• Screenshot

Fudged by reducing the textbox limit by a few characters. Everything on the serverside is treating the field as being 1024 characters long so I think it might actually be dependent on the input, e.g. if characters are URLencoded to take up more space or something odd like that. Might be worth further investigation but for now, closed.