Hello @RichardThorpe 

That is odd behavior - normally if the annotations were disabled for the assignment none of the tools should be working. If it's specifically the highlight only and it works sometimes and not others - It is likely your internet browser and we recommend clearing the cache and cookies or trying another browser.

https://community.canvaslms.com/t5/Canvas-Basics-Guide/How-do-I-clear-my-browser-cache-and-cookies/t...

I recommend trying some basic trouble shooting steps - different browser, clear cache, reset device, etc. If none of that appears to fix it, it would be best if you contact Canvas Support and they can take a look at the specifics of your course and assignment and provide better insight. 

https://community.canvaslms.com/t5/Canvas-Basics-Guide/How-do-I-contact-Canvas-Support/ta-p/389767