When probe chains are dispersed in a network or higher molecular weight matrix,the expectation from the reptation model is that the terminal relaxation function will narrow, due to suppression of constraint release. Previous experiments found no support for this prediction, although they relied on time-temperature superpositioning to obtain spectra. These studies also made use of polyisoprene (PI)/1,4-polybutadiene blends, whose thermodynamic miscibility is problematic We report herein measurements acquired with a broad-band dielectric spectrometer of the terminal relaxation of PI in blends with polybutadiene having substantial 1,2-content. This mixture is thermodynamically miscible, thus excluding the possibility of phase separation. Corroborating earlier work, we find that the experimental behavior is qualitatively opposite to the prediction. Suppression of constraint release broadens the terminal dispersion, increasing the deviation from the theoretical relaxation function. This suggests that the common assumption that reptation and constraint release are independent processes is incorrect.