Steps to reproduce:

Go to a site with a simple html select element, such as:
https://www.w3schools.com/tags/tryit.asp?filename=tryhtml_select

This is native HTML and as simple as you can get:

<select>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="opel">Opel</option>	
  <option value="audi">Audi</option>
</select>

Use tab and shift tab until the combo box has focus.

Expected behavior:

NVDA reads something to let me know that I activated a combo box and what the current value is.

Actual behavior:

NVDA is silent

System configuration:

NVDA version:
2017.3

NVDA Installed or portable: Installed

Windows version:
Windows 10 - 15063.540

Name and version of other software in use when reproducing the issue:
Firefox 55.0.3

Other questions:

Does the issue still occur after restarting your PC?
Yes

Have you tried any other versions of NVDA?
Yes, whatever was current on Feb 17 2017