A pump should not lose prime when it is off. So the only time you should see air out of the returns on start up is if you have solar. On solar drain air fills the lines and that needs to be purged on startup again. However, that should stop after a few minutes. So if you don't have solar, then it is likely an air leak on the suction side of the pump.

Also, to correct a misconception, when a pump cavitates, it does not produce air out of the returns. It produces water vapor in the impeller which then quickly collapses within the impeller as pressure rises so no net air is produced out of the pump.