Jail limits on FreeBSD 12.2, but not on 9.2

POST HEAVILY EDITED: actually the limitation is not about FreeBSD 12.2

in our lab we have a few HP Proliant DL360g8 with 32 GB RAM that we use to run jails.
Each server runs ~6000 simple jails that have only sshd and syslogd as running services.

They currently run FreeBSD 9.2, but I'm planning to reinstall them with 12.2.

While doing some tests, I noticed that if there are more than maybe 4000 jails using the same NIC,
the whole server gets really slow, although there is no clear error message in console or in the system log.

While a solution seems to be simply splitting the load among more than one NIC, I'm curious to
know where the limitation come from.
From my tests is seems that assigning 6000 aliases to a single NIC is ok, and only when starting all
the jails associated to those aliases that the slowness appears.