First of all, let's delete the line with the while, because it actually does nothing
The problem you code has, is that the seq.remove will shorten your list, which you are working with in your for-loop. So when you remove 1, the for loop wants to use the next number in the list, but because it got shortened, it will go skip one number.
I will try to explain it to you:
The for loop uses some kind of counter, so for i in seq, will say i = 0 in the first run.
So what the for loop does, is to use seq[i], so i will count the rounds.
Usually it would go i = 1, i = 2 and so on.
But when reach the 5 in your list, i is 2 at the moment, so even if you remove the 5 from the list, the counter is on 2 still, so it will go on with 3 at the next round.
That makes seq, but in the round of 1, you remove the 1, so the list is shortened, but the counter is still the same.
Let me show you on an example:
list_1 = [4,6,7,9,13]
for i in list_1:
if i%2 == 1:
so the result would be, that it removes the 7 on round 2 (counting starts at 0), so next round will be 3.
list_1 = 13, because your current list is [4,6,9,13]
Instead of removing the numbers, add the non-prime numbers to a new list and return it.
new_list = 
for i in seq: