RFID antenna beam direction and how to control it
For RFiD readers, Circular polarization antennas allow the reader to read tags in various orientations within the sensing range of RFID reader. This does not mean the RFID is generating RF power from all directions like a dipole antenna.
What you are seeing is potentially caused by the RFID reader set to a higher output power, which can cause several reflections and cause the reader to detect tags which are not supposed to be detected.
Some vendors set the RFID reader RF power at maximum by default. The accompanying configuration software(which varies by vendor) can allow you to reduce the power output and that should help you limit the field of view of the reader.
If reducing the power limits your reader's range equally in front and back direction. the reader's antenna could have an obscure propagation pattern focusing the radiated power in a certain direction which would not be an easy fix.