The issue described below has been fixed in Jira 8.12 – see Resolved Issues | Resolved with Jira 8.12 and Jira Service Desk 4.12 for details.
When Atlassian released Confluence 7.0, among other libraries, the Java mail library was updated to a newer version. Unfortunately, it turned out that this version had an issue that made it impossible for S/Notify to properly work any more. It took a while until the problem was finally fixed with another update of the Java mail library in Confluence 7.5.
Jira was still using an old version of the Java mail library that needed an update, too. So in Jira 8.10, Atlassian updated it to the version that was known not to have the issue like in Confluence 7.5. Good job!
However (you knew there would be a however), they also included the problem version – maybe from an earlier update of the library, then forgotten after the second? We don’t know, but now this leads to strange and unpredictable effects. For example, S/Notify sometimes works and sometimes doesn’t, depending on which library has been loaded – we haven’t seen a pattern yet.
Of course, we’ve reported this to Atlassian as soon as we had identified the problem cause.
Let’s hope we’ll get a fix very soon. In the meantime, better don’t upgrade to Jira 8.10+ if you don’t need to. We’ve had a report that the mail handlers may not work reliably any more, even without S/Notify installed.
Update: They have fixed the issue in Jira 8.12 – so just avoid Jira 8.10 and 8.11 and you're good!