At https://github.com/Islandora/islandora_openseadragon/blob/7.x/js/djtilesource.js#L66 the logic that sets the default for the tile overlap defaults to "2". If tile overlap is set to "0", which w the default used previously and the one that gives better results. as
See Comments from github pull request that introduced this tiny little bug
Giancarlo reported this
William Panting confirmed the bug
Nigel Banks found the error and will make a pull
Also, de default set to 2 at https://github.com/Islandora/islandora_openseadragon/blob/7.x/islandora_openseadragon.module#L10 could be revisited?