Upon further searching on issues, this looks like the same issue as https://www.drupal.org/project/simplesamlphp_auth/issues/3057851 → , where the SETENV variable in httpd.conf is being ignored.
In my production instance running 10.2.5, I fixed this with
$settings['simplesamlphp_dir'] = '/var/simplesamlphp';
in my drupal settings.php.
However, when I put this in my 10.3.6 site, I get a fatal error having to do with twig processing:
Twig\Error\SyntaxError: An exception has been thrown during the compilation of a template ("Attribute "twig_callable" does not exist for Node "Twig\Node\Expression\FilterExpression".") in "core/modules/node/templates/field--node--title.html.twig". in Twig\Environment->compileSource() (line 549 of /var/simplesamlphp/vendor/twig/twig/src/Environment.php).
Thinking that maybe there was a TWIG mismatch ("twig_callable" will be removed from TWIG 4.0, but this shouldn't be an issue for either the app or the module), I upgraded simplesamlphp to 2.3.2, and now the Federated Login button in simplesamlphp redirects back to /user.
Still need help.
This suggests and incompatibility with SimpleSAMLphp 2.0.