Thanks, Jon! That gives me somewhere to start looking. This is what I’ve found:
The 20fps video appears to be exactly 20fps as per Win Media Player (not a QT encoding). The other file is reported as 28.95fps by Quicktime Player when Premiere claims it to be 30fps. Interpreting it as 28.95fps makes it out of sync in the preview.
Rendering the first clip as 30fps in a new project looks to be a pain, but I’ll do it if that will work. Won’t recompressing it be unnecessarily lossy? I guess that’s secondary to getting this in-sync right now.