Previously, calling \Drupal\search_api\Query\Query::sort() multiple times with the same field ID (but, potentially, different orders) would override previous calls, leaving only the last call effective. This was directly contrary to how it works in the DB layer, and would also generally be assumed correct: that only the first sort on a given field has any effect and all others can be ignored. The potentially unexpected behavior was also undocumented, making it even more confusing, and also caused issues to site builders, when placing multiple sorts in a view (e.g., one exposed and one not).
#2880239: Exposed sorts, that also exist as a default sort for a view, never apply via the exposed form changed this (undocumented) behavior to the one consistent with the DB layer – that is: ignoring all subsequent sorts on a field after the first.
