Is it worth double-checking your wp-config.php level settings one last time:
define(‘MEMBERSHIP_LEVELS’, 10);
if you had quotes around ‘10’ it would not get set because there is a numeric check in the code.
If your define is ok…any chance wp-config is accidentally being cached. Are you on shared or managed hosting and with whom?
If you can push it to a staging site you can check the MEMBERSHIP_LEVELS via debugger or logfile.or debug plugin.