The chiral smectic phases of calamitic liquid crystals, SmC^{*} and SmC_{A}^{*}, are characterized by the synclinic ferroelectric F ordering and the anticlinic antiferroelectric A ordering in adjacent layers. Various states with mixed A and F orderings are degenerate at the frustrated phase-transition point. The degeneracy lifting is commonly caused by the long-range interlayer interactions (LRILIs), producing a series of biaxial subphases specified by a relative ratio of both orderings, q_{T}=[F]/([A]+[F]). Sandhya et al. [Phys. Rev. E 87, 012502 (2013)PLEEE81539-375510.1103/PhysRevE.87.012502] established, however, the importance of thermal fluctuations in the degeneracy lifting in some binary mixtures of MC881 and MC452. They observed the most intriguing interplay of thermal fluctuations and LRILIs in the stabilization of an apparently single subphase. Since no other detailed experimental study of the subphase has so far been made, we carry out its dielectric investigations and clarify the following five points: (1) the subphase is surely a single phase from ≈80^{∘}C down to room temperature; (2) the imaginary part of complex permittivity ε^{″} shows the weak antiphase mode and hence it must be antiferroelectric q_{T}=1/2; (3) ε^{″} becomes much stronger above ≈80^{∘}C, indicating the emergence of ferroelectric and/or ferrielectric states; (4) the dielectric amplitude gradually increases at least just above the 1/2 subphase, suggesting it be due to a continuous increase of q_{T}; and (5) at low temperatures the antiphase relaxation mode shows irregularities that indicate the important role played by the cooperative motion of the layer undulation as well as of the director tilting.