Hi Murray,
Welcome to Microsoft Community.
I'm Hahn and I'm here to help you with your concern.
It sounds like you've already done a lot of the right troubleshooting steps. The Surface Pro Flex Keyboard charging issue you describe appears to be the same issue reported by users in some communities.
Since this seems to be an ongoing problem, I have submitted this issue for you through internal channels. In addition, you can also submit your problem through the Feedback Hub (if convenient, please provide as much detail as possible, including pictures, repro videos, etc.).
Send feedback to Microsoft with the Feedback Hub app - Microsoft Support
Please share the feedback link after submitting, I will submit your feedback to the relevant team for investigation through internal channels.
Your Sincerely
Hahn. W - MSFT | Microsoft Community Support Specialist