I'm still fighting with my ppp issue here https://forum.opnsense.org/index.php?topic=15187.0
Now I switched from mpd to ppp to see if the problem shows up there as well. Turns out it does but ppp seems to handle it much better.
I now have a working ppp config for my Sierra Wireless LTE modem & everything works fine, even autostart with an improvised rc script plus NAT handled by ppp. However, I can't seem to log to syslog. By default ppp should log there and I enabled logging accordingly in the ppp config. In syslog conf I also see entries for ppp but they never show up in either syslog or a separate log.
I had a look at the log config options but not sure what should happen with a non-standard service like mine, is there anything special I have to do to get logging working for ppp?
Turns out it does log, to /var/log/ppps.log, not sure why though as this is not what is configured in syslog.conf.