The SONAME changed from 2.0.0 in 1.7.0 to 3.0.0 in 1.7.1, so other software linking against it should be rebuilt automatically and the sub-SLOT change can facilitate that.
More exactly, sonames are respectively "libldns.so.2" and "libldns.so.3", so SLOT="0/2" and SLOT="0/3" would make sense. $ scanelf -RSy /var/tmp/portage/net-libs/ldns-*/image TYPE SONAME FILE ET_DYN libldns.so.2 /var/tmp/portage/net-libs/ldns-1.7.0-r2/image/usr/lib64/libldns.so.2.0.0 ET_DYN libldns.so.3 /var/tmp/portage/net-libs/ldns-1.7.1/image/usr/lib64/libldns.so.3.0.0
Thanks for reporting. I will have a look very soon
Fixed in 1.7.1-r1