I was last using "Drupal8 W3CSS Theme 8.x-1.x-dev (2017-Aug-05)" and up until then the layout of the Search Form was the usual: the input field with the Search button right next to it on the right. See the attached image Search-Before.jpg.
After upgrading to "8.x-1.8 released 24 August 2017" the layout now has the two switched: the Search button comes first followed by the input field to its right. See the attached image Search-After.jpg.
Fixed
1.8
User interface
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It's likely this issue predates Contrib.social: some issue and comment data are missing.