Update to this, it's been sent to the R&D group via a support case. Apparently they're able to reproduce this issue in both W18 and Summer 17 orgs, however I had this working in Summer 17 - and the switch to W18 broke the functionality. Since they're saying it's not a regression issue, it's taking a lower priority.
For anyone at Salesforce looking at this, my case # is 17249103
I'm waiting to see if they're going to make a Known Issue of it for tracking.
Edit: This was fixed in a patch release.