Chemically B-site disordered PbScl/2Tal/2O3 relaxor crystals have been studied by means of dielectric and acoustic emission methods in the vicinity of Tm≈280 K. Along with the intrinsic cubic-to-rhombohedral phase transformation at Tc=261 K, a phase transformation at Tn=293 K is evidently detected by acoustic emission. Under an external dc electric field Tc gradually increases with increasing the field, whereas Tn exhibits nontrivial behavior, namely, initially decreases and then starts increasing, similarly to Tc. The characteristic temperature Tn is attributed to the occurrence of incommensurate modulated antiferroelectric regions, which have previously been suggested to coexist with both the cubic paraelectric and rhombohedral ferroelectric phases in the vicinity of Tm. The nontrivial behavior of Tn under an external dc electric field is also discussed.