When adding an interface to a blueprint, only blueprint created interfaces are available. Engine / custom interfaces written in code are not listed.
Yes - custom interfaces are present in 4.15.2
Working 4.15.2 (CL 3416026)
Broken 4.16 Preview 2 (CL 3415565)
Tutorial_BP_Interface is the only available interface to add
Engine interfaces are listed and can be added to the blueprint
Head over to the existing Questions & Answers thread and let us know what's up.