Well I've made a little bit of progress . . .
I went out today with the GPSMap62 and saved the Current track 4 times with the default date/time names -
Track_2020-02-01 101349
Track_2020-02-01 102350
Track_2020-02-01 103202
Track_2020-02-01 103533
Back home I copied the files to the PC and deleted all saved tracks on the device.
Then I made copies of the files and kept the originals un-edited. I loaded the copies into Basecamp and renamed them in various ways and kept exporting each version but did not edit anything else in the tracks, just the names. So I ended up with files like "01 Out", "02 Here", "03 There", "04 Return" and "AA Out", "BB Here", "CC There", "DD Return" with each file having the same embedded track name (as it appears in Basecamp) and file name.
Then I copied the files back to the GPSMap62 in various ways to try to determine what changes affected the track list order on the device.
When I copied the 4 original files back they only appeared in sequence if they were copied one at a time in the sequence required. Copying the earliest file first, then the next earliest then the next one then the most recent file last resulted in the correct order just like when they get saved on the device. If the files are copied in the reverse order they appear in the track list with the oldest track at the top and the newest at the bottom. If the files are copied back out of sequence that's how they appear in the track list.
Also the alpha/numeric embedded names of the tracks have an influence on the track list order.
I also renamed some of the files with nonsense names but left the embedded track names. These tracks still appeared in the track list correctly.
So part of the criteria of the device firmware for the track list order seems to be the track name (not the filename) and the sequence the files get copied to the device.
However, I've tried the same process with the files I downloaded for the Camino de Santiago trip but they are still showing out of sequence. Maybe it's because those tracks don't have any date/time and elevation data with each track point.
More testing and tea required . . .