To my astonishment I learned that after you purchase and apply a 50% offense/defense power enhancer for your units, if you attempt to apply any other enhancer (e.g., the 20% city defense enhancer), the 50% one goes away and cannot be gotten back. 

Is this a bug? 

Is it a tricky little landmine we are meant to discover via trial-and-error after the money has already been flushed down the toilet?