Mirror Wrap Mode in UI Images

Hey folks,
I wanna mirror some of the UI sprites in the project. Thus, I believe I'd reduce the using sprite atlas size almost by half.
In order to do that, I used the Sprite Wrap Mode to Per-axis and U axis to Mirror in the Sprite Import Settings (because of horizontal mirroring) Then I set the UI Image Type as Tiled. When I do that, my expectation was that the Sprite will tile itself by mirroring and will create a shield shape.

https://imgur.com/6gFyzVp
https://imgur.com/eAvARDA

But I couldn't succeed, unfortunately. The warning says to use Wrap Mode to Repeat. But my intention is to make the Sprite mirror. Not repeat, not slice. Am I missing something here?

Note #1: When I set Sprite Mode: Polygon (instead of Single), looks like it works in the scene until I press the Apply button. Then, it fails again.
Note #2: Unity 2018.4

 

Any ideas on this?