I can reproduce this in tip as well:

>>> ast.dump(ast.parse('a = (1,2)'), include_attributes=True)
"Module(body=[Assign(targets=[Name(id='a', ctx=Store(), lineno=1, col_offset=0)], value=Tuple(elts=[Num(n=1, lineno=1, col_offset=5), Num(n=2, lineno=1, col_offset=7)], ctx=Load(), lineno=1, col_offset=5), lineno=1, col_offset=0)])"
>>> ast.dump(ast.parse('a = (1 + 2)'), include_attributes=True)
"Module(body=[Assign(targets=[Name(id='a', ctx=Store(), lineno=1, col_offset=0)], value=BinOp(left=Num(n=1, lineno=1, col_offset=5), op=Add(), right=Num(n=2, lineno=1, col_offset=9), lineno=1, col_offset=5), lineno=1, col_offset=0)])"

The actual column number is lost when calling into 'ast_for_testlist'
from the 'LPAR' case of 'ast_for_atom'.