Already done, but no any progress with it yet. Ticket is here: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=272319 Connect problem - is not single question. MPD5 also crash with this PPP/TCP configuration on daemon stop. I think that kernel crash question - mostly related to kernel team, and ability mpd5 to response to new TCP connection - mostly to MPD5 developers. Problem of }}}}}} can be just because MPD5 not take "connect" event confirmation in PPP code from NG level, like it do with PPTP...
Also tested mpd5-5.8_7 with FreeBSD 12.0 - same problem. Look like it is related to new NG and it was started in kernel shipped with FreeBSD 12.0. FreeBSD 11.* work fine with same mpd5 versions.
mpd5-5.9_16 and mpd5-5.9_13 tested - they have same effect.
PPP over TCP not work on FreeBSD 12.0-13.2