miktex (lualatex.exe) fails to run from flask subprocess

I am trying to build a latex file using a subprocess call in a flask web-app.

    def pdf(self, ixTestsheet):
        build_dir = mkdtemp(prefix="testsheet_")
        tsfile = NamedTemporaryFile(dir=build_dir, suffix=".tex", delete=False)
        tsfile.write(self.latex(ixTestsheet, _print_type).encode("utf-8"))
        old_dir = os.getcwd()
        # os.chdir(build_dir)
        latexname = os.path.basename(tsfile.name)
        tsfile.close()

        # added this to check user of subprocess
        dirout = subprocess.run(
            ["whoami.exe"], cwd=build_dir, check=True, capture_output=True
        )
        current_app.logger.info(dirout)
        try:
            subprocess.run(
                [
                    current_app.config["LUALATEX_EXE"],
                    "--interaction=nonstopmode",
                    latexname,
                ],
                cwd=build_dir,
                check=True,
            )
            # call a second time to get a full build with page numbers etc.
            subprocess.run(
                [
                    current_app.config["LUALATEX_EXE"],
                    "--interaction=nonstopmode",
                    latexname,
                ],
                cwd=build_dir,
                check=True,
            )
        except subprocess.CalledProcessError as e:
            current_app.logger.error(build_dir)
            current_app.logger.error(e)

If I run the process from the flask app it fails with the following error. If I run the command from an interactive python session it completes.

[2020-12-04 11:51:13,512] INFO in testsheets: CompletedProcess(args=['whoami.exe'], returncode=0, stdout=b'<DOMAINNAME>\\<USERNAME>\r\n', stderr=b'')
lualatex.exe: No suitable temporary directory found.

Sorry, but "c:/Program Files/MiKTeX 2.9/miktex/bin/x64/lualatex.exe" did not succeed.
FATAL: No suitable temporary directory found.
FATAL: Data:
FATAL: Source: Libraries\MiKTeX\Core\Session\miktex.cpp:79
[2020-12-04 11:51:13,745] ERROR in testsheets: C:\Users\USERNA~1\AppData\Local\Temp\testsheet_oreb8wg5
[2020-12-04 11:51:13,745] ERROR in testsheets: Command '['c:/Program Files/MiKTeX 2.9/miktex/bin/x64/lualatex.exe', '--interaction=nonstopmode', 'tmpfk6un0c7.tex']' returned non-zero exit status 1.

I'm trying to figure out why MiKTeX is not able to find a temporary directory when run from flask, but can when run from an interactive session.

From what I can see:

1. The user running the command from the flask app is the same as the user running the command from the interactive session.
2. MiKTeX has all the packages it needs to build the latex file
3. The subprocess command isn't the problem
4. Permissions on the temporary directory seem to be fine

It seems both TMP and TEMP existed in the subprocess env, however TMP was set to an empty string. MiKTeX was using TMP before TEMP so it failed.

resolved it like this

subprocess.run(
                [
                    current_app.config["LUALATEX_EXE"],
                    "--interaction=nonstopmode",
                    latexname,
                ],
                cwd=build_dir,
                check=True,
                env=dict(os.environ, TMP=os.environ["TEMP"]),
            )