I am using Guard and an Authenticator extending the AbstractFormLoginAuthenticator.
I am logged in and edit my own user account in an user administration form and submit.
Some constraint fails (e.g. a required field left blank) - I get the form with error message, but am logged out. Therefore the next request redirects me to the login page.

I used the (almost) same code in a former project with Symfony 3.3 where this bug did not occur.
(only difference now is the new supports method to be implemented, which former was a part of getCredentials)