Gnugo’s play style is “I’m going to play away now, and you should answer me”, and when you start going “Hey gnugo those several tenukis are going to cost you this 40 point group, and I’m going to kill it” then you’re playing a game it handles poorly.
So unless the rank where you start tenuki-ing regularly, in particular when you think you can strongly attack or kill a group, is somehow a constant over time, then maybe gnugos rank will be constant.