The FCIDUMP "standard" originates from Peter Knowles' full configuration interaction (FCI) program, published in Comput. Phys. Commun. 54, 75 (1989).
The orbital active space for FCI is defined in terms of spin-restricted orbitals, since the exact FCI wave function is able to describe spin polarization of the density.
Extremely few programs support FCI calculations with spin-unrestricted orbitals, exactly since it is hard to make sense of an active space which differs for spin-up and spin-down electrons. There is simply no FCIDUMP standard for UHF orbitals. You don't need UHF orbitals, as RHF or ROHF orbitals suffice to get all states in exact theory.
Addendum: Many programs have introduced their own extensions to FCIDUMP, thus breaking compliance with the standard. Introduced for FCI calculations in 1989, the FCIDUMP standard is not optimal for e.g. coupled-cluster calculations which typically employ spin-unrestricted orbitals. FCIDUMP is also ASCII only, while binary i/o is orders of magnitude faster and requires less storage. An alternative is TREXIO, which is a new library designed for passing data like basis sets, orbitals, and one- and two-electron integrals between programs.