Thanks for reaching out to us, I will report this issue to product team for investigation and fix.
Let me double click on the issue you report -
For en-AU spelling check, colour and specialise are identified as an error. We will check with product team for this issue. Will let you know once there is any update.
At the meantime, have you tried proof mode? Does it work better?
Please let us know if you see any other issue for en-AU, thanks a lot.
Regards,
Yutong