This release changed the way of thumbnail assignment to make it 100% predictable which thumbnail will be displayed in case of multiple categories being selected for the post.
On post's publish / update the plugin gets the list of categories selected for the post. This list consists of category objects being sorted by lowercase category name and category id in ascending order ( the hierarchy of categories is ignored ). In next step the plugin traverses this list checking if the category id does exist in category / thumbnail mapping. If it does, we have two scenarios based on the value of "Do not overwrite if post / page has thumbnail assigned already" flag.
the record in postmeta table for the post with "_thumbnail_id" key is being created or updated
if the record in postmeta table for the post with "_thumbnail_id" key does not exist then a new record is being created otherwise no action.
In short: if the flag is off then the thumbnail for the last category in the list will be used, if the flag is on and there is previously assigned thumbnail then there is no change, if the flag is on and there is no previously assigned thumbnail then the thumbnail for the first category in the list will be used.