This is a known issue - we have it listed on our GitHub issues page here: https://github.com/nvaccess/nvda/issues/7396

At this stage, NVDA won't read the highlight normally.  NVDA does have an experimental advanced feature which will read it.  If you open NVDA's options.  Press NVDA+n to open the NVDA menu, then p for preferences and S for settings.  Press END to jump to the "Advanced" category.  TAB to the "I understand that changing these settings may cause NVDA to function incorrectly and check that, then tab to "Use UI Automation to access Microsoft Word document controls when available" and enable that.

This option uses a different method to get information about text in Microsoft Word.  It does note the highlight background colour correctly, however it may cuase other features not to work correctly.

Kind regards

Quentin.