I implemented a widget using the ARIA roles grid, row and gridcell. Inside of the gridcells I have <button>s where one of them has tabindex="0" and the others have tabindex="-1". I implemented keyboard navigation with arrow keys using a roving tabindex. Now when the focus lands on the button with tabindex="0" and I try to use the arrow keys while NVDA is on, it seems to have switched to browse mode because the arrow keys navigate between text lines and characters rather than going to another button in a different gridcell, as my code would do. This happens in Chrome and Firefox.
I'm basically following the example on https://www.w3.org/TR/wai-aria-practices-1.1/examples/grid/LayoutGrids.html#ex2_label for which this is broken as well. I'm also using a grid in a different place but there the gridcells don't contain buttons but have the tabindex themselves. In this case it works fine, NVDA seems to stay in focus mode.

Steps to reproduce:

1. Open Chrome or Firefox in Windows.
2. Browse to https://www.w3.org/TR/wai-aria-practices-1.1/examples/grid/LayoutGrids.html#ex2_label.
3. Click into the "Send To:" field, then press the right arrow key.
4. Notice how "Recipient Name 1" is focussed.

Expected behavior:

Focus should move to the "X" button. This works when NVDA is off.

Actual behavior:

NVDA reads the character "e". With each press of the right arrow key it reads another letter of "Recipient Name 1".

System configuration:

NVDA version: 2018.2
NVDA Installed or portable: Installed

Windows version:
Windows 10 Pro Version 1803 Build 17134.81

Name and version of other software in use when reproducing the issue:
Chrome version 67.0.3396.62 (64 bit)
Firefox Quantum 60.0.2 (32 bit)

Other questions:

Does the issue still occur after restarting your PC?
Yes.