I have multiple, identical openwrt devices, using mwan3, with WAN, Wifi-WAN, WWAN. WWAN with ppp. Although all devices identical, few of them have serious issues to acquire an IP from Verizon. It may take hours of retries. The "good" routers get the IP more or less immediately. Having got some info, that Verizon is very sensitive regarding "unexpected" traffic, there might be a race condition with pings etc. from mwan3, before an IP acquired, but after the modem connection was succesful. Is this possible ? Which depends on, when mwan3 starts to probe wwan.
If there is no IP/mask/gateway acquired on an interface, then the ping will not be sent anyway.
First you can declare in mwan3 if the interface is considered online or offline when mwan3 starts. If it is configured as offline, traffic will not be sent to it, but connectivity check will be tested in order to mark the interface as online.