Interesting thing just noticed today...
When clicking on Activities and looking at just the list of runs which defaults to the most recent activity, if I simply clicked a column (specifically Time or Distance) it would sort just fine, but I noticed one particular run was missing.
I was not filtering these results at all, so decided to do a simple filter - date range and distance. The result I wanted still didn't appear.
I did a text search, ok... there it was.
I did a new filter search adding type of activity (race) and ok now it appeared.
I checked the activity to see what was missing. It had everything but the "comment" filled out. After I added a simple comment it began to appear it all sorting with and without simple filtering. Adding the comment 'fixed' the issue.
I came to the forum to try to find this answer and in the meantime did all of the above and figured it out... so I thought I would post this so that either a developer might see it or it could help out someone else.
Happy trails...