For anyone else facing the same issue, I fixed it.
PS C:> DISM.exe /Online /add-capability /CapabilityName:MathRecognizer~~~~0.0.1.0
Deployment Image Servicing and Management tool
Version: 10.0.18362.1
Image Version: 10.0.18363.628
[==========================100.0%==========================]
The operation completed successfully.
It's possible that maths recogniser was uninstalled from Optional Features on my pc - I am not sure when, how or why - but it's back in any case.