The col_offset is actually correct when there is a decorator:

In [26]: code = '@decorator\nasync def f(): pass'

In [27]: tree = ast.parse(code)

In [28]: tree.body[0].col_offset
Out[28]: 0

The same issue appears with async for and async with:

In [31]: code = '@decorator\nasync def f():\n    async for x in y: pass'

In [32]: tree = ast.parse(code)

In [34]: tree.body[0].body[0]
Out[34]: <_ast.AsyncFor at 0x7f5cb6a77198>

In [35]: tree.body[0].body[0].col_offset
Out[35]: 10