This is happening because many of the registered meta-data keys start with _ to hide them from the WordPress custom fields box, but without an explicitly set auth_callback they are assumed to protected, causing __return_false to be used.
This should be solved by adding a proper auth_callback. I.e., __return_true.
We've only recently begun to use register_post_meta(), that's why it didn't show before. (Only reason to call register_post_meta() is allow block developers to work with this custom field. It's not needed for the plugin to function.)
Also, on my sites, I use a filter to show "hidden" custom fields, may be why I haven't experienced this.
See also https://bbpress.trac.wordpress.org/ticket/3280
This should be solved by adding a proper
auth_callback. I.e.,__return_true.We've only recently begun to use
register_post_meta(), that's why it didn't show before. (Only reason to callregister_post_meta()is allow block developers to work with this custom field. It's not needed for the plugin to function.)Also, on my sites, I use a filter to show "hidden" custom fields, may be why I haven't experienced this.