When several search results are returned whose relevance and defaultSortOrder field match (a series with many volumes with the same publication date, in my case), it would be desirable to have these results ordered by some deliberately chosen field, or by a sensible default (title alphanumeric desc would I think would be correct in all cases, potentially negating the need for an additional syspref). In my testing (with Elasticsearch), I believe the current secondary sort order to be [biblionumber asc, plus some randomness from a race condition introduced by the multiple concurrent processing threads used by the Elasticsearch reindex task, if multithreading was used].
screenshots of an example search with records in semi-random but almost biblionumber order https://imgbox.com/muwIxR4O https://imgbox.com/TO0X2pYG
I thought we had fixed the biblionumber not sorting numerical... but maybe not? I remember the intention to use it as the second sort criteria and I think we would like that as an option at least. It would allow to put the newest (highest biblionumber) first. Which version are you testing with? There is also: Bug 23875 - Elasticsearch - When sorting by score we should provide a tiebreaker