== Update ==
The subscription in question has now expired and disappeared from my application's subscriptions list (timestamp, PST: 3/22/2019 8:45:22
AM).
I will no longer be able to test against it (sorry S.O. posterity!).
However, good news: now that the problem child (subscription) in question is gone, I was able to create a new subscription and restart
my affected service. I am able to successfully submit get / update /
delete requests against any newly created subscriptions.
As this post is almost a year old and was most likely resolved via a fix by MSFT (no actionable fixes in our implementations), I am marking this post as "answered".
@baywet Thank you for the suggestion to do so.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…