Good integration between PR and ME for captions
I've encountered a strange bug. I have captions and an After Effects lower third on two different tracks on a timeline. Using Mac OSX 10.13.4, AME 12.1, Premiere 12.1.
If I render out of Premiere, no problems.
If I send to AME and try to render, the caption track renders as "Media Offline".
If I delete all content such that only the caption and the AE clips exist, the caption track renders as "Media Offline".
However, if I delete the AE content (turning off visibility doesn't do it) then the captions will render normally in AME.
It doesn't matter where on the timeline the two are; if both the captions and the AE clip exist together in the sequence, the captions will render as "Media Offline" in AME. Very weird.
Manual fixes are difficult because I have 40 or so timelines to render.

We have completely redesigned the captioning workflow in Premiere Pro version 15.0. You will see many dramatic workflow improvements. Please update to the latest version.
Learn all about the new captions workflow here: https://helpx.adobe.com/premiere-pro/user-guide.html/premiere-pro/using/working-with-captions.ug.html