The Provides: (for shim and shim-signed) and Obsoletes: (for shim-signed) that were added in the 13-0.3 build don't actually work:

[adamw@xps13k tmp]$ rpm -qp --provides shim-x64-13-0.3.x86_64.rpm 
warning: shim-x64-13-0.3.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID f5282ee4: NOKEY
bundled(openssl) = 1.0.2j
shim-signed-x64 = 13-0.3
shim-x64 = 13-0.3
shim-x64(x86-64) = 13-0.3

[adamw@xps13k tmp]$ rpm -qp --obsoletes shim-x64-13-0.3.x86_64.rpm 
warning: shim-x64-13-0.3.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID f5282ee4: NOKEY
shim < 13-0.3

I believe this is because (due to the define_pkg macro) they come after the subpackage description, which I think kinda 'closes' the subpackage metadata block. I think these Provides: and Obsoletes: would be applied to the main package (shim-signed), except we don't actually produce that package, so they just kinda...disappear into the ether. But I haven't actually checked this against the RPM documentation (hah) or whatever, it's just my guess.

This is a clear Fedora 27 Beta blocker, because it breaks UEFI installs: anaconda expects to install the package 'shim' for UEFI installs (through the bootloader class stuff), and there is nothing to provide 'shim', so it just dies.