What jumps out at me is you’re running a version of the agent that is over four years old. That version of the agent doesn’t even support async code. Further, it was built on the .NET 3.5 framework and has some significant issues with TLS v1.1, v1.2, and v1.3. This would be a good time to seriously consider upgrading.
IMPORTANT: Upgrading to the current version will result in some immediately apparent changes. There have been a ton of improvements with transaction naming, so you will likely see transaction names you are not used to. As well, the agent used to track database calls and external requests outside the context of a transaction, which was a bug. That has been fixed. The tracking of async methods in a transaction can cause changes in the charts that might be confusing at first. More detail about that can be found here:
It would be a good idea to read through the release notes for all of the releases since the one you are on:
Finally, I want to address the Azure side of things real quick. @stefan_garnham is definitely on the right track in asking what Microsoft change might have affected this. We see this issue with App Services all the time. A previously reporting application will suddenly stop reporting with no discernable difference. However, when we look at the settings for the application instance, we find there was an “Azure restart event” that took place. This may have been as inconsequential as moving the instance to a new platform, but the result is often the agent quits reporting. An instance restart typically solves this, though. That said, you may want to review the Diagnose and solve problems section in the Azure portal virtual machine menu for the system(s). There may be something in there that would explain why this happened when it did.