Thank you for pointing it out--that was the issue! Went to the plugin's page and found the solution written down at the bottom:
"If you add the hook for email registration to your theme in 2.6+, the loginas functionality will break. I added the following to the hook function in auth.php* to ignore admins logged in as students.
if (\core\session\manager::is_loggedinas()) {
$SESSION->mcautoenrolled = TRUE;
return true;
}"
*auth.php refers to the /auth/mcae/auth.php folder for the plugin.
Thanks all! You were a big help.