FWIW, I'm seeing something very similar to this with noVNC console on PVI 5.1-51 and Ubuntu 18.04 (bionic) VM with all updates.
When attempting to login on the noVNC console, I can type the username at the "hostname login:" prompt, but as soon as I press Enter, and start typing the password, extraneous Enter is pressed (so that if I type quickly, I see part of the password below the Password: prompt before the next login prompt. At that point, the login and password prompts act like they're receiving characters even though I don't type anything.
This repeats until the getty respawns the login.
An 18.04 box installed 80 days ago (and idle since -- without any reboots) doesn't exhibit this problem.
An 18.04 box installed just now does. I have tried "default", "vga", and "spice" for the Display type.
Both VMs are on the same hypervisor. I have not tried upgrading PVE yet.
If it matters, the VM displays this error on boot (the 80-day-old VM doesn't): "error: no video mode activated". This may not be related.