Getty (agetty, mingetty, others) need to run in their own process group. This used to be provided using chpst (which was, indeed, not the correct way to do this). A potential solution would be to prefix all the execs in getty run files with setsid, however a much simple solution would be to add -P to runsvdir's arguments in /etc/runit/2.
This is fixed in 2.1.2-r1. Thanks for the report.
