Rendering screws up track matte.

Can you 'round-trip' it through Ae via DL and render?

(I have not tried DL to Ae w/track mattes yet)

You're talking Timeline render not export, correct?

With a client behind me I would quietly dupe the segment,

DL to Ae, hit render and get approval.

If there are tweaks, tweak the original, re-dupe and repeat.

Clean up the mess later.

edit:

As for why it won't work to begin with, I'm stymied.

Are the Rampant masks white on black mattes or do they have alpha channels?  Have you tested with Track Matte effect on both Matte Alpha and Matte Luma?  Also, have you tried to do the same effect with a different matte (maybe just a white on black text matte).  I'm just wondering if it's an issue with the elements you're using. 

Something else to consider... you said you had 23.98 clips in a 29.97 timeline?  If the Rampant clips don't match your timeline, try changing their frame rate under Interpret Footage.

You automatically delete your preferences when you get unexpected Pr behavior, right?

The Pr Track Matte is horrible in the first place.  I don't use it.  I really dislike how the matte scales with any video you use it with.  That's not the way they should work.  I use Ae for all track matting, as they work properly in that app.

Anyway... sorry that didn't fix it.

I tried to reproduce your problem creating a similar avi mask in AE and importing it to Premiere.  Looked fine when setting it up, was screwed up when I rendered (in a way similar to yours).  Then, I put a background clip on a layer beneath the matted clip, re-rendered and it was fine.  Try dropping a layer of black beneath your matted shot and see if that works.

Probably just the way PP handles an empty channel (ie, not well).  Almost seems like it's showing it as premultiplied before rendering then unpremultiplied after.