OnTriggerEnter does not work propperly

I am having a trouble with OnTriggerEnter with a lot of objects.

I have a NavMeshAgent moving randomly throught the scene, then I have a sphere collider inside a children to detect nearby objects. If the object has an allowed tag it adds to the objects_near List.

It starts working right, but when it is working 1 or 2 minutes it starts dropping the objects_near variable to a wrong value, sometimes to 0, then, a few minutes later it drops to 0 permanently.

Is it a Unity bug? Is it my fault because I am using a lot of resources?

https://imgur.com/toLIMew

Code (CSharp):

1.         void OnTriggerEnter(Collider col)
2.     {
3.         string tagname = col.gameObject.tag;
4.    
5.         for (int i = 0; i < tags_allowed.Length; i++)
6.         {
7.             if (tags_allowed[i] == tagname)
8.             {
9.                 objects_near.Add(col.gameObject);
10.             }
11.         }
12.     }
13.  
14.     void OnTriggerExit(Collider col)
15.     {
16.         string tagname = col.gameObject.tag;
17.    
18.         for (int i = 0; i < tags_allowed.Length; i++)
19.         {
20.             if (tags_allowed[i] == tagname)
21.             {
22.                 objects_near.Remove(col.gameObject);
23.             }
24.         }
25.     }