WordPress 7.1 took down a client’s editor last week. Every custom ACF block moved its fields to the sidebar, and on one site they stopped saving with no error at all.
Not an ACF bug. One line of core JavaScript.
Through 7.0.x the editor iframed the canvas only if every block in the post was API v3 or higher. A default ACF block is v2, so one of them kept the whole canvas un-iframed. That was the escape hatch. Most of us never knew we were standing on it.
In 7.1 disableIframe does not appear in a single file under wp-includes/js/dist/, and editor.js passes shouldIframe: true as a literal.
Ten-second check:
grep -rl "disableIframe" wp-includes/js/dist/
Enter fullscreen mode Exit fullscreen mode
4 files means you are on 7.0.x. 0 means your ACF fields are moving.
Updating ACF does not fix it, and V3 replaces inline editing rather than restoring it. Full post-mortem with the core diff and the fix.
What did your grep return?