I believe I've found the source of the problem. I had added dummy-notebookutils as a requirement in the setup.py of my external library. Therefore, when I imported said library into Synapse, the dummy-notebookutils was added to the sparkpool, which interfered with the built-in mssparkutils object.

Now that I've removed dummy-notebookutils from setup.py and tried again, the issue seems to have disappeared.