Humanoid avatar resets to strange pose (not default pose)

I have a problem that I've come across before in that humanoid characters will start to reset to a strange pose (screenshot attached)
https://www.dropbox.com/s/mp7zg9x2bvz7lz0/Screenshot 2019-08-24 20.44.17.png?dl=0

This happens part way through projects.
It doesn't has any effect on the animations in the animator for the character but for my recent project it is necessary to fix for use with IK tracking.

Is there a way to reset to the default avatar pose?

I've seen somewhere else where someone was able to fix this by adding a controller with a Tpose animation, recording (in the Animation editor) then Starting, then pressing the record button again but this method doesn't work - the avatar just resets to this hovering pose

Any help would be appreciated.