To reproduce:

0. Make sure your cursor is nowhere near the tab suggestion box
1. Type in /team option someTeam (with a trailing space)
2. Hit tab twice - color will now be the chosen value (in the chat bar)
3. Hit the down arrow thrice - nametagVisibility will be the highlighed value (colored yellow in the tab suggestions box)
4. Hit tab - seeFriendlyInvisibles will be placed in the chat bar instead of the value I just manually highlighted from the suggestion box (seeFriendlyInvisibles is the value after the one I had highlighted)

Note that step 3 can also be done with your cursor. Expected behavior would be for step 4 to fill the highlighted (yellow) value into the chat bar.

18w06a partially fixed this issue, but selecting a different value by moving your cursor still is affected.