Sometime between 1.4-rolling-202301210856 and 1.4-rolling-202302110748, VyOS has started rejecting PPPoE usernames containing a '.' character.

In the UK at least, convention amongst several ISPs is to use PPPoE usernames of the form ID@domain.tld.

[edit interfaces pppoe pppoe0]
malcolm@kylerhea# set authentication username "XX12345@giga.net.uk"



  Username is limited to alphanumerical characters, -, _, #, and @ with a total lenght of 128
  Value validation failed
  Set failed

(the error message also contains a typo: "lenght"!)