I may be wrong, but it looks like dub that bundled with dmd in tar.xz archives is built with earlier dmd version than the one included in the same archive. If it's the case, this should be fixed. Phobos and dmd may have fixes that affects dub too, so it should be built against the latest standard library. E.g. check this issue https://github.com/dlang/dub/issues/1096
Looking at the source (https://github.com/dlang/installer/blob/master/create_dmd_release/create_dmd_release.d#L532), it seems like this was done on purpose: // build dub with stable (host) compiler, b/c it breaks // too easily with the latest compiler, e.g. for nightlies Though I agree that the released dub should use the freshly built Phobos.
Ideally dub should be built with a stable LDC though...