✅ Why does my list get zeroed?

public class Solution {
    public IList<int> FindAnagrams(string s, string p) {
          List<int> items = new List<int>();
          int i = 0;   
          while(i < s.Length){ //loop through the s which is longer than p
            if(p.Length == s.Substring(i).Length && p.Contains(s.Substring(i))){ //edge case where s and p are of the same length
               items.Add(p.Length);
            } else if(p.Contains(s.Substring(i))){
                
                    items.Add(p.Length);
                
            }
            
            i++;
        } 
        return items;  
    }
}