Enable packet filter firewall (pfctl -Ef) temporarily stops all network connection
Pf is a stateful firewall. It means it strictly accounts current state of connections and if you temporarily turn it off and then on again its internal view could become outdated so that some affected connections would stall and never recover.

There's a workaround that man pf.conf describes "…


  If stateless filtering of individual packets is desired, the no state keyword can be used to specify that state will not be created if this is the last matching rule.  A number of parameters can also be set to affect how packet filter handles state tracking.  See STATEFUL TRACKING OPTIONS below for further details.


…"

But new ruleset loading with pfctl -f should not cause any troubles unless you're flushing states specifically, for e. g..