namespace package sorts differently when using the `from namespace import x` form

Given a configuration file like this:

[tool.isort]
import_heading_firstparty="First party"
import_heading_thirdparty="Third party"
known_first_party = ["namespace.mymodule"]
default_section = "THIRDPARTY"

And an input file like this:

import namespace.mymodule
from namespace import mymodule

Installing isort[pyproject] (==4.3.21), I get:

# Third party
from namespace import mymodule

# First party
import namespace.mymodule

Normally this would not be a problem because you can put "namespace" into known_first_party, but when the namespace is backports or some other shared namespace, there are both first- and third-party packages in there. It seems to me that in the case of from x import y, the sorting should be based on x.y, not on x.