During boot, Ubuntu uses the "details" plugin to display the entered password prompt and the bullets. The trouble is that in initramfs the cryptroot script makes the password prompt _two_ lined. The "details" plugin clears only the current line and then overwrites the password prompt string on the cleared line. The effect of this is that the _second_ line in the password prompt always gets overwritten. Thus on every key input, the first line overwrites the previously displayed second line (printed due to the previous key press). Thus you see the first line repeated over and over again with every keypress.
The solution to this could be one of the following:
1) make the password prompt single line
2) clear the whole screen instead of clearing a single line before writing the password prompt.
3) append the bullets instead of the line
4) clear in some way the previous line.
IMHO, the disadvantage of 1,3,4 is that in case you boot with plymouth:debug, the password prompt would still be repeated (as you will have debug messages interleaved and what you clear is different than what you intended to). Clearing the screen would on the other hand cause a clearing of screen with every keypress and wipe out any debug messages that you could be interested in.
Shall soon upload a patch with a appropriate solution.