Wanted to share this info after researching the issue a bit further.
When initially calling the mentioned API method, the results are as follows, despite there being known subscriptions present
Notice the @odata.nextLink property - I have found that following these links repeatedly as they are provided in response does eventually get you to your subscriptions. Note that for me to see them I had to go through 5-10 nextLinks, and at times continuing to skip through empty pages, but I do eventually find the subscriptions.
This seems more like a workaround and I definitely still believe this to be a bug in Graph, as the nextLink should not be necessary unless there is a full page of results already returned. Hope this helps others.