Plugin that registers new embed handler cannot get editor preview in core, but can with Gutenberg plugin

[notnownikki]

This seems to be an issue with plugins that use wp_embed_register_handler to add more embed handlers.

I tested the plugin at ​https://wordpress.org/plugins/soundfaith-embed/ which provides an embed handler for faithlife.com

If you use the plugin version of Gutenberg, the editor works fine and the default embed block provides a preview of the embed, as expected.

If you just use core WordPress, the oembed proxy returns a 404.

Tested with 5.0.2, embedding ​https://sermons.faithlife.com/sermons/334709-the-gift-of-peace

[swissspidy]

Duplicate of #45447.