Invalid value. Matching delimiter not found - github-actions

Question

In updating GitHub actions to reflect the recent announcement deprecating set-output, I have run into the following error attempting to send multiline output to GITHUB_OUTPUT following the provided documentation
Error: Unable to process file command 'output' successfully.
Error: Invalid value. Matching delimiter not found 'e8e24219e2b73f81'
Below is the example action:
name: Action Test
description: test new action output
runs:
using: "composite"
steps:
- name : write
run : |
delimiter="$(openssl rand -hex 8)"
echo "OUT<<${delimiter}" >> $GITHUB_OUTPUT
cat test.json >> $GITHUB_OUTPUT
echo "${delimiter}" >> $GITHUB_OUTPUT
shell : bash
id: write
- name: Print Output
run: echo ${{ steps.write.outputs.OUT }}
shell: bash
In theory this should generate a random delimiter, put it at the beginning and end of the output, and allow the action to then print the multiline file. In practice, I'm unsure what is happening to the second instance of the delimiter as there is no match.
I have tried various solutions such as those posted in this topic

Answer 1

This turned out not to be the issue at hand for the asker, but would lead to the same error message, so leaving it here:
The JSON file is missing a newline at the end, so the actual contents written to the output file look something like
OUT<<eabbd4511f4f29ab
{"key":"value"}eabbd4511f4f29ab
and the closing delimiter can't be found because it's not at the beginning of a line.
To fix, we can apply this answer to add a newline if it's missing:
run : |
delimiter=$(openssl rand -hex 8)
{
echo "OUT<<$delimiter"
sed -e '$a\' test.json
echo "$delimiter"
} >> "$GITHUB_OUTPUT"
shell: bash