The answer is in the PEP of the generator expressions, in particular the session Early Binding vs Late biding:
After much discussion, it was decided that the first (outermost)
for-expression should be evaluated immediately and that the remaining
expressions be evaluated when the generator is executed.
So basically the array in:
x for x in array
is evaluated using the original list [1, 8, 15] (i.e. immediately), while the other one:
if array.count(x) > 0
is evaluated when the generator is executed using:
print(list(gen))
at which point array refers to a new list [2, 8, 22]