Solr formatFilter assumes query arguments passed are correctly formed into something Solr understands. Happens that since this is passed as a query argument, people could have modified it, or even reached the GET argument length limit getting a cropped query.
That makes at: https://github.com/Islandora/islandora_solr_search/blob/7.x/includes/results.inc#L530-L533
results in no split at all. Subsequent calls to the second element of that array end in warnings like
"Notice: Undefined offset: 1 in formatFilter()..."
To test: run something like this (on a real repo)
and look at your Drupal logs.
Other consequence of this is that you end with an empty element in the query breadcrumb, which makes me wonder if this could be even a security issue? Not filtering, sanitizing that input?