I recently enabled multisite (possibly a coincidence) and shortly after noticed simplesearch stopped working (always returned 0 results). Looking at the plugin config in the debug panel I discovered that the filters setting is an array containing both my value in user/config/plugins/simplesearch.yaml and the value in the plugins default yaml file (which is 'category: blog'). This broke my search because I do not have a blog category in the site's taxonomy. Removing the filters: key from user/plugins/simplesearch/simplesearch.yaml (my multisites are symlinked here) resolved the issue.
As far as I know the correct behavior is for my config to override the plugins default config not result in a merged array both both configs. Am I missing something or is there some kind of bug here?