Hey there! You're probably reaching an internal timeout of some sort, such as the ExecTimeLimit value, which is 3600 seconds (6 minutes) by default.
If the .sql file is large, you could easily exceed that time limit with normal work. You can manually edit the /usr/local/cpanel/base/3rdparty/phpMyAdmin/config.inc.php file to change this, although cPanel will overwrite those changes as part of the nightly update on the system. The top comment noted
here explains how you can make those edits more permanent if this is a change you'd like to see stay in place on the system.