First just to clarify, the POE status does not affect or restrict the transmit power of the 303H radios.
The transmit power value value that is (set or) reported in the UI is in terms of total EIRP, which means it includes MIMO gain and antenna gain. The antenna gain number is a peak number. Since the actual pattern fluctuates, the power in a particular direction could be a couple of dB lower. When seeing 18dBm in the UI, it is very well possible that the actual power in a particular direction is 15dBm or lower.
At 5GHz, the "free space pathloss" is about 67dB when 10m (30ft) from the transmitter. In a more realistic environment, that loss can be significantly higher. Assuming 75dB, you would expect to see -60dBm 10m removed from the AP (LOS).
Your receive antenna may have negative gain, introducing some more loss.