A temporary workaround hack is:
root@OpenWrt:~# cat /usr/lib/ModemManager/connection.d/10-report-down-and-reconnect
#!/bin/sh
# Automatically report to netifd that the underlying modem
# is really disconnected and reconnect if interface was up
# require program name and at least 4 arguments
[ $# -lt 4 ] && exit 1
MODEM_PATH="$1"
BEARER_PATH="$2"
INTERFACE="$3"
STATE="$4"
[ "${STATE}" = "disconnected" ] || exit 0
. /usr/share/ModemManager/modemmanager.common
. /lib/netifd/netifd-proto.sh
INCLUDE_ONLY=1 . /lib/netifd/proto/modemmanager.sh
MODEM_STATUS=$(mmcli --modem="${MODEM_PATH}" --output-keyvalue)
[ -n "${MODEM_STATUS}" ] || exit 1
MODEM_DEVICE=$(modemmanager_get_field "${MODEM_STATUS}" "modem.generic.device")
[ -n "${MODEM_DEVICE}" ] || exit 2
CFG=$(mm_get_modem_config "${MODEM_DEVICE}")
[ -n "${CFG}" ] || exit 3
IFUP=$(ifstatus ${CFG} | jsonfilter -e '@.up')
logger -t "modemmanager" "interface ${CFG} (network device ${INTERFACE}) ${STATE}"
proto_init_update $INTERFACE 0
proto_send_update $CFG
[ "${IFUP}" = "true" ] && ifup ${CFG}
exit 0
But @aleksander0m laid out a proposal on what needs to be done to address this properly.