This started happening to us recently after working for many years. Judging from this page it's possible that they stealthily made this no longer work unless you have a special enterprise contract.
Please note that the $og_image_url social preview feature is reserved for paid Branch accounts with a dedicated contract. If you do not have a paid Branch account, you will not see this feature in the Branch Dashboard.
This FAQ page was "Updated about 1 month ago", and the other FAQ pages were updated 3 years ago.
The only way we can customize it now is by specifying a $fallback_url that already has the desired og_image_url in its metadata, which isn't always ideal.