We studied the proton-rich T_z=−1 nucleus ⁷⁰Kr through inelastic scattering at intermediate energies in order to extract the reduced transition probability, B(E2;0⁺→2⁺). Comparison with the other members of the A=70 isospin triplet, ⁷⁰Br and ⁷⁰Se, studied in the same experiment, shows 3σ deviation from the expected linearity of the electromagnetic matrix elements as a function of T_z. At present, no established nuclear structure theory can describe this observed deviation quantitatively. This is the first violation of isospin symmetry at this level observed in the transition matrix elements. A heuristic approach may explain the anomaly by a shape change between the mirror nuclei ⁷⁰Kr and ⁷⁰Se contrary to the model predictions.