Drupal8 CiviCRM conflicts with Masquerade module
Reported on d.o originally by jptillman (who is on chat.c.o with the same name) in the Masquerade queue: https://www.drupal.org/project/masquerade/issues/3019665
I confirmed the same issue, and also confirmed that uninstalling CiviCRM resolves this issue. This fits a pattern of Civi having some sort of incorrect interaction with the D8 user login - see #53 (closed), core#231 (moved), and CRM-19878 for other examples.