That’s a good question. However, it requires defining what one means by an “open shell singlet”.
Namely, if you run the calculation you supplied, you’ll see that the wave function you get from CASSCF is not single-reference; you have two major configurations in it that contribute 95% and 4% of the norm. Systems like this one that are not single-reference are often called open-shell singlets.
By the way, looking at the CASSCF wave function, you can actually see that you have the wrong reference state: the SCF converges to
Final Occupation by Irrep:
A1 A2 B1 B2
DOCC [ 2, 0, 1, 1 ]
whereas in the CAS wave function this configuration only has the 4% weight. If you set
A1 A2 B1 B2
DOCC [ 3, 0, 0, 1 ]
then you’ll get a much lower Hartree-Fock energy: -38.87720372911416 instead of -38.75428141573684, i.e. a decrease of 0.123 Hartree which is pretty huge.
Now, the second alternative for an open-shell singlet, which typically refers to SCF level calculations is dependent on broken spin symmetry: the alpha and beta orbitals have different spatial parts.
In principle nothing would prevent you from treating these cases with high-level ab initio methods. However, since basically all CASSCF codes are formulated using spin-restricted orbitals, you can’t feed in symmetry broken orbitals. The main reason for this is that defining an active space would be very difficult, if the spatial parts could be different; your alpha and beta spaces might be totally different.
Still, broken symmetries are typically just artefacts caused by the lack of proper treatment of electron correlation in the wave function. Going from RHF to UHF may give you lower energy, because you’re freeing the opposite-spin electrons to avoid each other. But, if you include opposite-spin correlation, which is missing from Hartree-Fock, in your treatment like in CASSCF, then you probably won’t get spin symmetry breaking anymore.