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.
So be kind to your receiver and don't put a 24 dB gain preamplifier in front of it. For those of us who live in the city we have too much man made noise already and in many cases you are only going to make it worse by including a high gain preamplifier. Keep the gain down to 12 to 15 dB and some of your problems will be prevented. Double shielded cable is a must. Low insertion loss is also important.