To my knowledge nothing in tunnels should do this, there is one possibility I can think of but its not tunnel related directly, and that is that your link out to yor first hop isn't as strong as it could be, if it isnt registering 100% LQ it's already slightly degraded, if your connecting to a large tunnel group you will be adding extra routing traffic, which while minimized, is still a constant traffic increase on your network taking up more RF time, the more hidden nodes you have the more effect this broadcast activity has. Wifi broadcasts are by their nature highly error prone so ETX often increase while directed traffic can better coordinate itself.
Thats just a guess not knowing exactly what your layout and links look like.
Thats just a guess not knowing exactly what your layout and links look like.