Thank you @GeorgiaC ! Good answer.

Is it because this thread has "solved" written in the header that Helpline staff think its actually solved? We think the thread should be labelled "under investigation"

So howabout, add to the name of this thread, text along the lines of (Contact QB helpline and quote ref  INV-82727 if you are experiencing this issue, to let us know and you will be added to the cases. Not all customers are affected.). Or alternatively instruct staff to reply to comments on this thread using the above text, to prompt people to phone in so QB gets a better idea of the scale of the problem, otherwise your customers will think (as I did initially), if they post on here, QB will listen and resolve the issue within a couple of weeks which patently isn't happening. We are almost up to 12 months now of waiting for a fix.