The last(?) update added the option to add tabs right next to the current tab, and I think it's pretty useful and nice.
However, when the option is enabled, clicking new tab button also adds a tab next to the current tab.
Example 1.

Although this "new tab" button is at the end of the list of all tabs, the new tab will open next to the current tab.
Example 2.

This "new tab" button opens a new tab next to the current tab, while Safari opens it at the most right position.
To wrap things up,
In Safari
- Pressing CMD+T adds a new tab next to the current tab
- Clicking the new tab button adds a new tab next to the last tab
In Orion (when Open new tab next to current tab option - enabled)
- Pressing CMD+T adds a new tab next to the current tab (Good)
- Clicking the new tab button also does this (?)
I think "new tab button"s and "CMD+T" should be configured separately.