1 pointThis tells me you have an actual directory named 2019-holiday-party inside your gallery folder. When I created a similar directory on my end then I get results similar to yours. The problem is mod_dir which in addition to handling DirectoryIndex files, I'm not sure exactly how it's interacting with your mod_rewrite rules, but it appears as though it's triggers based on the original request url instead of the re-written URL. When it issues the redirect however it still picks up the query-string that was added to the re-written URL. This trailing slash behavior can be disabled by setting DirectorySlash Off in your configuration. That should resolve the problem, though I'm not sure it's a good solution. Another solution would be to make sure your pretty URL's don't map to an actual directory on the server.
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