I noticed that observations that get new comments on them get the updated_at field changed, but they don’t seem to move up in the sort order when sorting by Date Updated.
In this screenshot, the 4th observation has
"updated_at":"2020-01-26T13:37:17.901-05:00", while the 5th observation has the more recent
"updated_at":"2020-01-27T14:00:45.352-05:00". Based on the timestamp of the comment on the 5th observation, it should actually be the first result in this search.