So as not to conflict with existing projects, I have a installed Magento 2.4.2 under the ddev Docker framework and have the Luna demo site working on the standard https port 443. Both the end user shop pages and admin sections display as expected.
Next I update the web/*secure/base_url to include a port number (https://ddev-magento2.ddev.site:8443/) and then follow that with a bin/magento cache:flush, the shop stays fine but the admin section does not allow anyone to get to either the login screen or get to the settings pages. The browser log shows a request for https://ddev-magento2.ddev.site:8443/admin_y312l0 followed by an "endless" loop https://ddev-magento2.ddev.site:8443/index.php/admin_y312l0/admin/index/index/key/2348840aae440e5986... The browser recognizes a redirect problem after about 20 requests to the same URL.
For someone with Drupal and other CMS experience, I would think it should be pretty easy to add a port to the base URLs but I must be missing something. Been digging in with Xdebug and am starting to think this is a bug.