To investigate a field-induced level crossing of the ground-state doublet in PrTr₂Zn₂₀ (Tr = Rh, Ir), we performed ultrasonic measurements in pulsed magnetic fields applied along the [110] and [001] directions and analyzed the results in the framework of the strain-susceptibility approach. Above 40 T for H || [110], we observed an elastic softening of the transverse modulus (C₁₁ C₁₂)/2 corresponding to the ground-state doublet. In both compounds the softening is followed by a minimum at about 47 T at low temperatures. We predict the presence of a new field-induced phase boundary in PrTr₂Zn₂₀ at this field with two possible cases. The magnetic field of the minimum cannot be explained by only the quadrupole interaction.