parallel-ssh sometimes redirects stdout to stderr

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Describe the bug
parallel-ssh sometimes redirects stdout to stderr in the output.

To Reproduce
Create a "hostfile" and an example script:

#!/bin/bash
set -ex
echo "Example"

Copy the files to all hosts, then run:

parallel-ssh -i -v -h ~/hostfile bash ~/example.sh 

Expected behavior
Nothing is printed to stderr.

Actual behaviour
We got something in stderr:

[1] 13:52:24 [SUCCESS] host1
Example
Stderr: + echo Example
...

Additional information
This is just one example, we have a few other cases where the stdout of the scripts are redirected to stderr of the parallel-ssh output.