Empty result of Printing itertools.combinations (applied on list of lists "L=[l_1,l_2,..,l_n]"), after changing one list (l_i) in L

Question

Given the following code:

from itertools import combinations

l_a = [1,2,3]
l_b = ["a","b","c"]
l_c = [0.1,0.2,0.3]

l_abc = [l_a,l_b,l_c]

l_com = combinations(l_abc,2)


print("l_abc t1 : ",l_abc) #t1
print("l_com t1 : ",*l_com) #t1 

print("--------------")

l_a.append(4)

print("l_abc t2 : ",l_abc) #t2 
print("l_com t2 : ",*l_com) #t2 

The output is:

l_abc t1 :  [[1, 2, 3], ['a', 'b', 'c'], [0.1, 0.2, 0.3]]                                                                                     
l_com t1 :  ([1, 2, 3], ['a', 'b', 'c']) ([1, 2, 3], [0.1, 0.2, 0.3]) (['a', 'b', 'c'], [0.1, 0.2, 0.3])                                      
--------------                                                                                                                                
l_abc t2 :  [[1, 2, 3, 4], ['a', 'b', 'c'], [0.1, 0.2, 0.3]]                                                                                  
l_com t2 : 

After the construction of a list of 3 lists (l_abc):

1. t1: When I print the result of itertools.combinations (l_com) in time 1 (t1), it gives the correct results.
2. t2: I modify one list (l_a) that is part of the whole list of 3 lists (l_abc). When I print again the result of the previous combinations, the results is empty.

However, you can see that, for printing only list of lists (l_abc) work fin in t1 and after the updating in t2.

Q1) Why is that, and
Q2) How I can get the correct combinations in t2 ? (naivly call again the combinations second time).

Thanks.