thanks for sharing the issue and the solution :)
I do not think that the issue is related to the intermine version.
The Autocomplete and SolrIndexHandler java classes (which are called
during the postpocess) only delete previuos indexes, see here:
not the cores which need to be created manually -only once- after
installing solr or any new version of solr.
Have you installed a new version of solr?
Please, let us know, if this happens again (hopefully not!)