I think I've figured out why.
Five of six sources used 'time of day' timecode. One starts at 00:00:00:00 (it was recorded on a BMD Video Assist, and it automatically starts recording if it receives timecode. Fine in some cases where it's used for backup, but in our case it would make more sense to let the operator start the recording as it was hooked up to an ancient Canon XLH1 miniDV camera... )
So what happened when I synced on timecode is the Assist footage starts at 00:00... and the rest is found 19 hours into the timeline. Resolve doesn't like that I guess.
I can sync the five first then add the last after.
I think bottom line is; I'd like the Assist to be able to receive time of day timecode from camera, but still have manual control of whether to start and stop recording.