Are you unable to reproduce with the same site in a different environment or in the same environment? - if different environments, are they using the same version of PHP and MySQL/MariaDB?
I would also look to see if there are any errors. - check the developer log in the CP - open up the Network tab in dev tools and try to save an entry. check the different requests responses to see if any contain an error. - try disabling all extensions and see if you can save an entry. If so, then enable one at a time till the issue is reproduced.
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