Some users are experiencing problems with WYSIWYG fields whereby the Visual tab doesn’t appear to initialize correctly, and where content added may not appear to be saved correctly (the Visual tab appears empty when a post is edited).
Switching to the Text tab shows that there is content in the field, and there are no issues saving content in the Text tab.
This issue can be difficult to reproduce.
It has been reported for many plugins (including ACF, for example), since WordPress 5.6, and appears to be related to the timing of code execution, which may vary from server to server.
It is reported to WP core several times (https://core.trac.wordpress.org/ticket/52111, https://core.trac.wordpress.org/ticket/52133, https://core.trac.wordpress.org/ticket/53644 is still open).
We await confirmation that the issue is fixed in WP.
Comments on the WP tickets suggest that using the jQuery migrate helper plugin and choosing the legacy jQuery version can “fix” the issue: https://wordpress.org/plugins/enable-jquery-migrate-helper