Parallels Desktop v12.1.3 (41532) / Windows 10 Intermittent loss of shortcuts in Excel 2016

Hi,
I have a client who's intermittently loses the ability to the the F4 function in Excel 2016. The problem is rectified by rebooting Windows.

The client has Parallels Desktop v12.1.3 (41532) with Windows 10 and Excel 2016 installed. The hardware is a MacBook Pro 2016 with Touch bar.

We have tried plugging in an external keyboard when F4 has stopped working but the same is true of the external keyboard too.

Below are a couple of Parallels support docs to which we have tried to no avail.

"F9-F12 function keys do not work as expected in Virtual Machines"
http://kb.parallels.com/en/116172

"How to enable accessibility for a specific application in OS X 10.9 Mavericks"
http://kb.parallels.com/116418


Any further suggestions or advice would be greatly appreciated.