pppd is a delicate flower. On orderly shutdown, it likes to tell the other side. This seems to take at least a second even when no real network latency is at play, on busy systems 1.5 seconds easily ends up being inadequate.
A violent shutdown is generally okay apart from that it can leave garbage (port lock) behind and the other side potentially confused for a while.
As it happens, this interacts badly with modemu.pl which is used for testing: the pseudo terminal in PPP line discipline mode has no idea that the remote disconnected and while ModemManager is learning that something wrong the hard way (AT command timing out, because the remote still expects to talk PPP), the test times out.
Let's increase the timeout to something more reasonable.