mjhjmtu

I'm running a ubuntu server installation with a tty console, and sshd terminal logins.
Both ssh keys and passwords work for authenticating over sshd.
Lately, on the console, when I try to log in in text mode, I get a curious behavior:
I enter my login name. It prompts for the password. However, it very quickly prints a newline, and whatever part of the password I hadn't typed already is echoed in cleartext to the console.
Then, it prompts for login:, but quickly times out and prompts for login: again ... 5 times, until it goes back to clear the screen, and re-issue the agetty process.
At that point it's "stable" and waits for a username again, but entering a username and pressing enter re-starts the "timeout and newline" behavior.

The systemd log for getty@tty1 says the following:

-- Unit getty@tty1.service has finished starting up.
--
-- The start-up result is RESULT.
Feb 08 12:12:47 xps15 login[1610]: pam_unix(login:auth): conversation failed
Feb 08 12:12:47 xps15 login[1610]: pam_unix(login:auth): auth could not identify password for [jwatte]
Feb 08 12:12:50 xps15 login[1610]: FAILED LOGIN (1) on '/dev/tty1' FOR 'jwatte', Authentication failure
Feb 08 12:12:51 xps15 login[1610]: pam_securetty(login:auth): cannot determine username
Feb 08 12:12:54 xps15 login[1610]: FAILED LOGIN (2) on '/dev/tty1' FOR 'UNKNOWN', Error in service module
Feb 08 12:12:55 xps15 login[1610]: pam_securetty(login:auth): cannot determine username
Feb 08 12:12:58 xps15 login[1610]: FAILED LOGIN (3) on '/dev/tty1' FOR 'UNKNOWN', Error in service module
Feb 08 12:12:59 xps15 login[1610]: pam_securetty(login:auth): cannot determine username
Feb 08 12:13:03 xps15 login[1610]: FAILED LOGIN (4) on '/dev/tty1' FOR 'UNKNOWN', Error in service module
Feb 08 12:13:03 xps15 login[1610]: pam_securetty(login:auth): cannot determine username
Feb 08 12:13:06 xps15 login[1610]: FAILED LOGIN (5) on '/dev/tty1' FOR 'UNKNOWN', Error in service module
Feb 08 12:13:06 xps15 login[1610]: TOO MANY LOGIN TRIES (5) on '/dev/tty1' FOR 'UNKNOWN'
Feb 08 12:13:06 xps15 login[1610]: pam_mail(login:session): cannot determine username
Feb 08 12:13:06 xps15 login[1610]: pam_unix(login:session): close_session - error recovering username
Feb 08 12:13:06 xps15 systemd[1]: getty@tty1.service: Service has no hold-off time, scheduling restart.
Feb 08 12:13:06 xps15 systemd[1]: getty@tty1.service: Scheduled restart job, restart counter is at 1.
-- Subject: Automatic restarting of a unit has been scheduled
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
--
-- Automatic restarting of the unit getty@tty1.service has been scheduled, as the result for
-- the configured Restart= setting for the unit.

I imagine the problem is the first two lines:

pam_unix(login:auth): conversation failed
pam_unix(login:auth): auth could not identify password for [jwatte]

Why would this happen, and how can I best debug it?

This turns out to be caused by: https://askubuntu.com/questions/1113704/tty-doesnt-wait-for-password

I don't have mojo points enough to mark this question to redirect there, but thanks to @PerlDuck for providing the answer!