Here is the behavior of Safari tabs work on macOS:
The behavioral mismatch is annoying. If I am working on a window with three tabs and I want to research something about the tab I am currently reading I want the new tab to open directly next to the current tab, generally.
To fix this you can use Automator, Apple Script, and Settings keyboard shortcuts.
tell application "Safari" tell front window set _old_tab to current tab set _new_tab to make new tab at after _old_tab set current tab to _new_tab end tell end tell
This is an update to a post on Daring Fireball using no additional software.