I actually think the GM is meant to be negotiated. It’s stated openly up front so players don’t actually have to math the dice, buts also stated openly so that when the GM says “that’s going to be hard” the player doesn’t think “oh, I thought you meant TN 16 hard…TN 18 is too hard.

So when you say TN 18 and the player says nope…that’s completely the way I understood the game to work…

So the player was telling you what their character wanted, and then when the character realized the Beorn wasn’t likely to agree he decided to push for it.

One might question whether backing off right then was or wasn’t in character (if he is a rash egotistical character it may have made more sense to roll anyway…and so maybe backing odd out of character might be a sign of turtling behavior)…but mechanically, that’s exactly how I think it’s supposed to work.