urgent help, please -- audio sync in 23.976 to 59.94 export

Facts are simple and I suspect solution is, too, but I can't figure it out. I shot 23.976 (4k DCI) and must deliver 59.94 (1920x1080) for broadcast, but audio is going out of sync on a final render. Why and how do I fix?

• video is 23.976 from P4k, edited in Project 1 with synced external audio at 96kHz/24-bit on 23.976 timeline
• exported separate 23.976 master video (Cineform MOV) and 96/24 audio (PCM)
• in new Project 2 (set to 59.94 project and timeline), I added master video and audio to 59.95 timeline. Playback on timeline start to finish (~1h) is perfect.
• rendered to MP4 (H.264) with embedded AAC audio for delivery. Audio drifts out of sync terribly.

I got the same results when I mastered to 29.97 (3:2 pulldown).

What am I doing wrong?