Custom keyboard selection does not survive power cycle

I have written a custom keyboard handler using the chrome.input.ime, it works fine and runs fine.

I select it as you would any other keyboard, i.e. Dvorak, and all is fine.

I let the Chromebook sleep / wake and all is still fine, my custom keyboard is still active.

However, if I power off the Chromebook, when I switch it back on it has changed to a different keyboard !, "GB".

Initially I thought it was my extension, so I tried some of the example keyboards available in git : https://github.com/google/extra-keyboards-for-chrome-os

But they suffer from the same problem. When you power cycle the Chromebook it looses the keyboard selection.

Any suggestions as to how to stop this happening ?

Where is this setting stored ? Could my extension alter this setting at startup and auto-select it's self ?

FYI - This app will be used exclusively through the googles managed app for schools and businesses.