We experience the same issue on WP 6.8.2, ACF Pro 6.5.0.1 and Firefox 142.0.1.
We tried to activate the checkbox to delay the initialization of TinyMCE (setting in the presentation tab of the WYSIWYG-editor-field). So now the editor is only loaded when you actually click on it, which seems to solve the issue in our case.
Well, that’s a bummer. I thought it’d always trigger before writing anything to the database. Then I’ll have to write another validation for the frontend like you suggested. Thank you!
