I thought you’d never ask.
I certainly did search and search before I posted my question. Here’s one for the archives.
Avid was reading the entire sequence as S resolution and exporting in that corresponding size because of 3 out of several titles created in the Title Tool. It wasn’t the Marquee Tool as I had suspected. As soon as I isolated them out, it returned to DV 720 size when exporting. It required micro-surgery because I had no idea what was triggering it, and not all the Title Tool creations were S resolution.
The S resolution titles shared this in common: shadow effects. For what that’s worth.
So, when that happens, something, anything is in S resolution. One frame in S resolution, apparently, will trigger the whole sequence to be interpreted that way.
That has been my experience. My micro-surgery gloves have been put to rest. Hallelujah.