Interesting! Seems to be caused by an issue in the AWS JS SDK itself.

This example:

const AWS = require("aws-sdk");
const client = new AWS.S3({endpoint: "http://s3.localhost.localstack.cloud:4566"});
(async () => {
  await client.createBucket({Bucket: "test"}).promise();
  const result = await client.getBucketLocation({Bucket: "test"}).promise();
  console.log(result);
})();

... results in:

$ node test.js
{
  LocationConstraint: '<LocationConstraint xmlns="http://s3.amazonaws.com/doc/2006-03-01/">'
}

(without the changes in this PR). Not really worth it creating a bug report there - too many of our reports/PRs have been ignored by AWS in the past already.. 😄

Coverage decreased (-0.02%) to 91.56% when pulling 0b08f15 on s3-xml-newline into 0b9f90a on master.

LocalStack integration with Pro

       3 files  ±0         3 suites  ±0   56m 57s ⏱️ + 5m 35s
   952 tests ±0     912 ✔️ ±0  40 💤 ±0  0 ❌ ±0 
1 165 runs  ±0  1 097 ✔️ ±0  68 💤 ±0  0 ❌ ±0 

Results for commit 0b08f15. ± Comparison against base commit 0b9f90a.

Hi! Can this change be impacting GetObject responses from S3? My XML files are suddenly receiving this newline after the preamble.

Thanks for the pointer @fabiob - mostly likely that was an oversight, so it's quite possible. We'll look into it shortly.. 👍