The ldpd template treats the ttl_security leaf as a dict, so ttl-security disable renders an invalid FRR command.
Location
[[ https://github.com/vyos/vyos-1x/blob/rolling/data/templates/frr/ldpd.frr.j2#L22-L25 | data/templates/frr/ldpd.frr.j2:22-25 ]]
[[ https://github.com/vyos/vyos-1x/blob/rolling/interface-definitions/protocols_mpls.xml.in#L80 | interface-definitions/protocols_mpls.xml.in:80 ]]
Problem
ttl_security is a leaf string; .disable is undefined.