It is happening cause your MacBook may not be able recognize the external microphone. The sound you are hearing is actually coming from the internal microphone.
It is happening because MacBook only recognizes TRRS jacks. (Old iphone headphone jacks.)
You can solve this problem by using an adapter that converts a regular jack (TRS jack) to TRRS jack. That also need to have a jack to connect the headphone separately.
At, first connect the external mic jack to the adapter. Then connect the TRRS jack to MacBook. Check the sound. Now you can hear the sound that is coming from the internal mic.
Now also connect the headphone to the adapter and again check the sound. Only now, you can actually hear the sound coming from the microphone.