For whatever reason, Feroxes are still super buggy, and these bugs mostly show up in Single Player (SP). I’ve put 500 hours into vanilla unofficial servers and never ran into this.

On the Fjordur map in SP, you can find Feroxes spawning way past the 150 max level cap inside the Ferox cave. Take the one I just ran into for example—it was a level 280, and it was perfectly tameable.

Also in SP, there's still a chance for them to dupe mid-tame. The cloned Ferox actually keeps the taming progress, and you can just finish taming the dupe as well.