Same issue here-
Upon saving nothing seems to be happening with the featured image box as normal.
-Tested for plugin conflicts by disabling all and setting to default 2020 wordpress theme.
–Issue still persisted
-Checked for thumbnail availability at https://www.get-youtube-thumbnail.com/
–Both default and maxres available.
-Changed https://i3.ytimg.com/vi/0JPXXAXxcuI/maxresdefault.jpg to img.youtube.com variant to check is img variant was depreciated
–both thumbnail URL strings work.
