In LogFormatterMathtext, we have

    else:
        if usetex:
            s = r'$%s%d^{%d}$'% (sign_string, b, nearest_long(fx))
        else:
            s = r'$\mathdefault{%s%d^{%d}}$'% (sign_string, b,
                                               nearest_long(fx))

But this is a problem if one is using non-integer base for a plot, like 1.5. Suggestion is to fix by %.2f^{%d} or something similar if a non-integer base is detected. Joint suggestion of Punarbasu Purkayastha, Karl-Dieter Crisman.