Editing the global php.ini at /usr/local/lib/php.ini does not increase values in the internal PHP for cPanel for PhpMyAdmin.

You have to edit the internal php.ini, which apparently it is reading from /usr/locafile.rather than /usr/local/cpanel/3rdparty/etc/phpmyadmin/php.ini file.

nano /usr/local/cpanel/3rdparty/etc/phpmyadmin/php.ini
; Maximum size of POST data that PHP will accept.
post_max_size = 50M
upload_max_size = 50M