Album and Album Artist are identical for all 69 files but Disc# wasn't! The culprit, it turns out. Never even paid attention to that field before.
Thanks so much.

What about Album, Album Artist and Disc#?

A Playlist may be better suited to maintain a specific order for a random set of files.

I've got 69 MP3 files (a "mixtape") that I've tried copying to my iPhone 6. "Track #"s are all correct and in order. However, 14 of the files do not end up in sequence in the iPhone. They all appear, somewhat randomly, after file # 69. All files have the same bitrate (128). Any ideas, anyone?