I have the exact same problem. It looks like this is a bigger bug then just for one website. Is this being worked on?
To clarify, not only the edits are not saved, it also seems that saved fields are not registered itself or got lost after a widget save.
Added: It seems that the following notice pops-up on ACF Pro 5.11.4:
Notice: acf_get_value werd <strong>verkeerd</strong> aangeroepen. <strong>Advanced Custom Fields PRO</strong> - We've detected one or more calls to retrieve ACF field values before ACF has been initialized. This is not supported and can result in malformed or missing data. <a href="https://www.advancedcustomfields.com/resources/acf-field-functions/" target="_blank">Learn how to fix this</a>.
Maybe this has something to do with the widgets now rendered in a “Deprecated widget” container?
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