Are you using a browser Extension?
• Some youtube extensions can do exactly that... pause a video while you watch another in a different tab..
• And other extensions can do the opposite.
So if you don't have the first extension to revert the setting, try installing the second extension to unlock the youtube setting.
https://chrome.google.com/webstore/search/youtube%20multiple%20video%20viewer%20extension?_feature=5stars&_category=extensions
I use this extension to make the video pause if I move to another tab (explained below)... it may help you unlock the Youtube setting.
you may have to enable the setting in the extension and later disable it*.
These are the settings I use in the "YouTube™ No Buffer (Stop Auto-playing)" extension:
(✓) Stop auto-buffering of embedded YouTube players
(✓) Allow JavaScript access for embedded YouTube players (some websites use JavaScript to start the player)
( ) Stop auto-buffering the player on play-list mode
(✓) Release no-buffer (start playing) when tab gets focus1
(✓) Pause video when tab is hidden2
*(Enable this setting, restart the browser and disable it)
1: If this option is checked and [2] is unchecked, YouTube player is released from no-buffer mode once.
2: If this option is checked and [1] is checked too, YouTube player gets paused when tab is hidden and it gets resumed when tab is active again.