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This is because the first line of the function definition was the line of the last decorator in 3.7, and it is the line of the first decorator in 3.8.
$ rlwrap ./python -m dis
@decorator
@decorator
@decorator
def func():
 pass
In 3.7:
 2 0 LOAD_NAME 0 (decorator)
 3 2 LOAD_NAME 0 (decorator)
 4 4 LOAD_NAME 0 (decorator)
 6 LOAD_CONST 0 (<code object func at 0x7f53a8281c40, file "<stdin>", line 2>)
 8 LOAD_CONST 1 ('func')
 10 MAKE_FUNCTION 0
 12 CALL_FUNCTION 1
 14 CALL_FUNCTION 1
 16 CALL_FUNCTION 1
 18 STORE_NAME 1 (func)
 20 LOAD_CONST 2 (None)
 22 RETURN_VALUE
In 3.8:
 2 0 LOAD_NAME 0 (decorator)
 3 2 LOAD_NAME 0 (decorator)
 4 4 LOAD_NAME 0 (decorator)
 2 6 LOAD_CONST 0 (<code object func at 0x7f7045a80100, file "<stdin>", line 2>)
 8 LOAD_CONST 1 ('func')
 10 MAKE_FUNCTION 0
 12 CALL_FUNCTION 1
 14 CALL_FUNCTION 1
 16 CALL_FUNCTION 1
 18 STORE_NAME 1 (func)
 20 LOAD_CONST 2 (None)
 22 RETURN_VALUE