Figured I'd share the solution I ultimately came up with.
Unfortunately, what I want isn't possible out of the box. But I did find a rather good file re-naming scheme:
Date - Time - Original File Name
You can bulk re-name all of your files rather quickly, and then sort by file name. Now your files will always display in the proper chronological order.
I still think Adobe can do a much better job of providing its users a smart (or better yet, highly customizable) sorting function! I find it surprising that the sort function is so primitive, given how powerful and flexible its filter function is.
I am also frustrated by this problem. When sorting burst by Capture Time, Lightroom puts files shot the same whole second out of order. I'm maintaining the serial file number from the camera. Sadly Lightroom isn't using it for the secondary criteria. I know that I could just sort by filename, but sometimes I shoot with two different camera bodies - which would result in them not "interleaving" properly.
A number of people have complained about the improper capture-time ordering of photos when the camera doesn't insert fractional seconds in the EXIF data. There is a specific feature request in the official Adobe feedback forum to use filename as the secondary sort criteria when sorting by capture time, but unfortunately I can't find it right now: Recently active topics in Photoshop Family about Photoshop Lightroom.
I think the best workaround (other than getting a camera that records fractional seconds) is Chrirobo's suggestion of renaming your files:
Date - Time - Original File Name
and then sorting by filename. This will handle switching between camera bodies. It will just hiccup every time the camera numbering rolls over from 9999 back to 1.
I see how that would work. However, I already have a system for file naming, and I'm not interested in renaming 31,000+ files. (stubborn choice, I acknowledge ). It just seems like Lightroom should be clever enough to have a secondary sort (filename!) when presented with multiple files taken the same whole second. As it is now, it seems pretty random. An example order for a single whole second: 5199, 5198, 5202, 5197, 5200, 5201. It feels like the files were shuffled like a deck of cards! They're not in size order either (0.3MB variance in file size in that burst). If it helps, the files are from a Sony a6000 (happens on my A7ii as well). Converting to DNG on import.
I found a feature request in the official Adobe feedback forum asking to sort first by capture time then by filename, to handle cameras that still don't record fractional seconds: LIGHTROOM - Sort by capture time shows files in wrong order. If you add your vote and detailed opinion to that topic, it will make it a little more likely Adobe might act on it. (But they've made very few changes to the Library over the last few years.)
You should also complain to Sony that they should be recording fractional seconds in the EXIF metadata!
billickc, I understand your point of view; I myself was nervous about the daunting task of having to rename so many files. Fortunately, Lightroom was able to rename 50,000 plus files for me in around 10 minutes; it did a great job consistently naming the files so that they automatically sort in chronological order, and Lightroom can always re-re-name the files back to their original name if you get nostalgic later
For me, it was a quick, easy, and reliable way to solve the problem.