Interesting. Mainly because there is, and has been, a workaround for operators that seemed to, in some very rare cases, send the IPv4 address via DHCP a bit late when DUAL mode is selected for the mobile data. We just increased the time to wait for IPv4 address to arrive from 2s to 5s in 4.5.0 as an attempt to help in these kind of situations. Maybe that isnāt the case with these operators.
So yes, you are correct, if the operator fails to provide IPv4 or the device for some reason does not acquire one setting the mode for mobile data to IPv6 only will eliminate such wait. It was actually a Finnish operator Elisa that had this kind of issue of late IPv4 delivery. Because of architectural restrictions the wait was the most feasible way of addressing the issue. But it could be improved to eliminate the wait completely - since the wait, as you noticed, will not set the mobile data connection as connected until the wait has completed OR an IPv4 address was received.