If you are having issues saving a JD Builder page with a lot of sections, it would be a server side limit.
If you are using a proxy service such as CloudFlare, it would return an error such as 524 HTTP error and the log file would display something like this.
[Tue Jan 05 15:54:34.305290 2021] [:error] [pid 225411:tid 47237830199040] [client 220.127.116.11:57382] [client 18.104.22.168] ModSecurity: Request body no files data length is larger than the configured limit (1048576). [hostname "www.joomdev.com"] [uri "/administrator/index.php"] [unique_id "X-TSCSZZR9gIj8rC10i6sQAAAAU"], referer: https://www.joomdev.com/administrator/index.php?option=com_jdbuilder&view=page&layout=edit&id=193
What this means is that the Content-Body limit in Apache is too low for the JD Builder JSON to be posted and stored in the database.
Contact your webhost and request them to increase the variable SecResponseBodyLimit to a higher number (the default limit is 1 MB and changing it to 2 MB fixed the issue for us) in the apache configuration.
The following cPanel Forum Post was helpful: https://forums.cpanel.net/threads/how-to-increase-mod_security-limit.659009/