why does collider data-type null itself?

i have a game-object (enemy = E) and two player-objects (P1 & P2). when leftclicking on a player-object and then rightclicking on a location on the ground-plane in the scene a marker is placed and the player moves to the set position,

E has a sphere-collider marked as trigger on it. when a player-object enters the trigger, the collider data-type is stored and the entered player becomes a target for E to focus on, i've tried to set it up in such a way that if P1 enters the zone first, then E will ignore P2 until P1 is dead, or leaves using a on-trigger exit.

now here comes my strange experience, if P1 enters the zone, and then collides with the box-collider that E has, then the collider-data that lets E locate P1 becomes null,

the problem with this is esentialyy that it would the player to simply ram their enemies to imobilize them, a cool ability but not something that should be a default feature.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.AI;
 
 public class EnemyController : MonoBehaviour
 {
 
     public float MeleeRange = 5;
     public float AttackStrength = 15;
     public float EnemyShield = 50;
     public float EnemyHealth = 400;
 
     private Collider EnemyTarget;
     public NavMeshAgent Agent;
     private Vector3 EnemyTargetDestination;
 
     private void Update()
     {
         if(EnemyTarget != null)
         {
             EnemyTargetDestination = EnemyTarget.transform.position;
             SetTargetAsDestination();
         }
         else
         {
             Debug.Log("no target");
         }
 
         if(EnemyHealth <= 0f)
         {
             Destroy(gameObject);
         }
     }
 
 
     private void OnTriggerEnter(Collider other)
     {
         if (EnemyTarget == null)
         {
             if (other.tag == "Player")
             {
                 Debug.Log("player entered range");
                 EnemyTarget = other;
             }
         }
     }
 
     private void OnTriggerExit(Collider other)
     {
         if(EnemyTarget.gameObject.name == other.gameObject.name)
         {
             EnemyTarget = null;
             EnemyTargetDestination = transform.position;
         }
     }
 
     private void SetTargetAsDestination()
     {
         if (Vector3.Distance(transform.position, EnemyTargetDestination) >= MeleeRange)
         {
             Agent.SetDestination(EnemyTargetDestination);
             Agent.isStopped = false;
         }
         if (Vector3.Distance(transform.position, EnemyTargetDestination) <= MeleeRange)
         {
             Agent.SetDestination(transform.position);
             DamageTarget();
         }
         transform.LookAt(EnemyTargetDestination);
   
     }
 
     private void DamageTarget()
     { 
     
             EnemyTarget.GetComponent<StatController>().TakeDamage(AttackStrength * Time.deltaTime);
             Debug.Log("Target health = " + EnemyTarget.GetComponent<StatController>().CharacterHealth);
     }
 
     public void TakeDamage(float damage)
     {
         EnemyHealth = EnemyHealth - (damage / ((EnemyShield + 100) / 100));
     }
 }
 

upon collision between P1 and E, P1 can step away by a unit or so, and then E.target == null, if P1 and E then come into contact again, P1 will become the target of E as long as they touch, and then E.target becomes null if P1 steps away again....

here is some screen-capture of the behaviour.

https://vimeo.com/367278329

EDIT: I've tried the same thing, but disabling the smaller box-collider, the box-collider does not affect the situation.