I've noticed that these lines are added at the end of /system/build.prop:
net.ppp0.dns1=8.8.8.8
net.ppp0.dns2=8.8.4.4
net.dns1=8.8.8.8
net.dns2=8.8.4.4
The entries in net.dns1 and net.dns2 are properly replaced with DHCP-provided information, but net.ppp0.dns1 and net.ppp0.dns2 are not unset...