It can be seen that virtually any signal can pass only slightly attenuated into your transmitter. Even if your transmitter has a 60 dB down second harmonic low pass filter it can easily pass signals below that in frequency, unattenuated. The isolator, a circulator with a dummy load, on the transmitter is like an RF diode. That is it exhibits directional characteristics. Your transmitter signal can go to the antenna but signals from the antenna can't get back to your transmitter.
Nothing is perfect, including isolators.
20 to 25 dB is normal for single stage isolators. Outside of this operating bandwidth they look like a 50 Ohm transmission line and once again allow out of band signals into your transmitter amplifier. It would be better if no harmonics came out of your amplifier. Preamplifiers can be a major source of problems. Many manufacturers produce amplifiers which have entirely too much gain.