If it is not possible to save new custom permalinks, the problem is probably in the size of the custom permalinks array. Unfortunately, I am not able to reproduce your server configuration, so I need you to send me two screenshots:
One with the size of custom permalinks array (you can find it in “Tools -> Permalink Manager -> Debug”)
One with “max_allowed_packet” and “PHP Memory Limit” values (you can find them under “Tools -> Site Health -> Info”):
Thanks, given the debug data, the problem seems to be somewhere else.
Could you possibly record a screencast video and show all the steps, so I can reproduce them on my dev server. Before changing the slug, please open a browser’s console log (there might be some AJAX/JS errors displayed that might be helpful in troubleshooting).