dhcp6c was unconditionally started with -D (debug level 2). In daemon mode setloglevel() only calls setlogmask() for levels 0 and 1 - level 2 falls through and leaves the log mask wide open, so every LOG_DEBUG message ends up in syslog:

dhcp6c[3935]: copy_option: set client ID (len 18)
dhcp6c[3935]: copyout_option: set identity association
dhcp6c[3935]: copy_option: set elapsed time (len 2)
dhcp6c[3935]: copy_option: set option request (len 4)
dhcp6c[3935]: client6_send: send solicit to ff02::1:2%wwan0
dhcp6c[3935]: dhcp6_reset_timer: reset a timer on wwan0, state=SOLICIT, ...

On a link where no DHCPv6 server answers this repeats (e.g. WWAN interfaces) for every SOLICIT retransmission, forever.

Drop the flag - errors and the parser diagnostics used by the T7058 smoketest are logged at LOG_ERR and remain visible.