You can simple insert this option, by editing template sudo nano /usr/share/vyos/templates/accel-ppp/l2tp.config.tmpl and then reconfigure l2tp service.
But better create bug report on the phabricator. This issue appears with MS-CHAP-v1/v2 and enabled CCP and for others protocols.
Note: CCP should be disabled by default. In CLI we only have the option to enable CCP.
https://phabricator.vyos.net/T2601