`--use-fine-grained-cache` for `dmypy` hangs

According to the docs (https://mypy.readthedocs.io/en/stable/additional_features.html?highlight=caching#caching-with-mypy-daemon) enabling the fine grained cache should help speed up initial mypy runs. However, this doesn't seem to be working, and dmypy seems to hang when using it:

# Slow initial run (mypy):
❯ rm -r .mypy_cache
❯ time mypy --cache-fine-grained .
mypy --cache-fine-grained .  82.04s user 9.06s system 97% cpu 1:33.69 total

# Faster cached run (mypy)
❯ time mypy --cache-fine-grained .
mypy --cache-fine-grained .  45.68s user 6.77s system 95% cpu 54.732 total

# Slow initial run (daemon)
❯ dmypy stop
❯ time dmypy run .
dmypy run .  0.32s user 0.31s system 0% cpu 1:23.13 total

# Faster cached run (daemon)
❯ time dmypy run .
dmypy run .  0.08s user 0.06s system 12% cpu 1.124 total

# Initial run using mypy cache (daemon)
❯ dmypy stop
❯ time dmypy run . -- --use-fine-grained-cache
# Nothing happening for at least 10 minutes...

Start by reproducing the documented caching workflow with dmypy run . -- --use-fine-grained-cache, comparing it with mypy --cache-fine-grained . and normal dmypy run .. Trace the dmypy fine-grained-cache entry point and determine why the initial run does not complete; done means the command finishes and uses the cache as described in the caching documentation.