What steps will reproduce the problem?
1.  Have reputation bar showing.
2.  Equip a legendary weapon.
3.  reputation bar disappears while legendary weapon is equipped.  Unequipping the weapon makes the bar show again.

What is the expected output? What do you see instead?
I think this is working as intended but I wanted to log it as information as there are a lot of 'reputation bar not working' tickets and this might help.

What version of the product are you using?
v4

Do you have an error log of what happened?
n/a

Please provide any additional information below.

I do not think this is a defect, but is caused by Blizzard.  When bartender is disabled, and the native UI is enabled, when you equip a legendary weapon, the reputation bar disappears and in it's place comes a bar called 'artifact power'.  If you go to the character sheet and make sure you have just selected a reputation to "show as experience bar" on the UI, it does not show.