What is change in internal energy of a system in which combustion occurs at constant temperature?
The change in internal energy of a reaction mixture is a function not only of the temperature change but also of the changes in the amounts of reactants and products in the mixture.  This is because energy is consumed and released when we break- and make chemical bonds.  The equation you wrote for the internal energy change applies only to a pure species or to a mixture of constant chemical composition.  The occurrence of chemical reactions causes a change in the internal energy even at constant temperature.