Bug - Adding a layer with transparency requires sequence render
Adding a clip with transparency (eg: An exported graphic from AE) onto track 2 (or 3, 4, etc), above a footage item, requires the timeline to be rendered before the transparency is visible. Without a sequence render, the graphic appears on a black background, as though an alpha channel is not present.
Alternatively, you can use the Screen transfer mode as a temporary workaround.
This is with the standard Lossless with Alpha export from AE (ie: Animation with alpha).
1) Render a graphic from AE (with alpha channel)
2) Add rendered graphic above a footage layer in Premiere
3) Admire the way that Premiere ignores the alpha channel until you render the sequence (or the clip)
Just for reference:
OS X 10.13.4
iMac (Retina 5k, 27-inch, Late-2014)
4 GHz i7
AMD Radeon R9 4GB