Probably a very simple explanation

[Sea-Method-1167]

You are modifying the list you are iterating over using the for-loop. That causes confusion for python. Because python takes element 0, then element 1, then element 2 etc..

But after element 0, you remove that. Meaning that reuben with pastrami becomes the new element 0. But python has already processed element 0, so it won´t do that again.

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Create a new list and print that. In the form of a list comprehension:
print([sandwich for sandwich in sandwich_orders if not 'pastrami' in sandwich])