E-Mail subject not showing utf-8 characters

[4lexvav]

Preconditions

1. Magento Version 2.3.0 installed via composer
2. PHP Version 7.1.25

Steps to reproduce

1. Create an e-mail template containing the "Ő" letter in the subject. For example, based on the Newsletter Subscription Success email template.
2. Change Success Email Template at the system Newsletter settings.
3. Subscribe for newsletter.
4. View the email in your email client. In my case, macOS Mail.app.

Expected result

1. Email subject should contain the "Ő" letter in its subject.

Actual result

1. Email subject contains question mark:

[magento-engcom-team]

Hi @4lexvav. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento-engcom-team give me $VERSION instance

where $VERSION is version tags (starting from 2.2.0+) or develop branches (for example: 2.3-develop).
For more details, please, review the Magento Contributor Assistant documentation.

@4lexvav do you confirm that you was able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

[magento-engcom-team]

Hi @engcom-backlog-nazar. Thank you for working on this issue.
In order to make sure that issue has enough information and ready for development, please read and check the following instruction: 👇

• 1. Verify that issue has all the required information. (Preconditions, Steps to reproduce, Expected result, Actual result).

  Details

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  Details

  - Add the comment @magento-engcom-team give me 2.3-develop instance to deploy test instance on Magento infrastructure.
  - If the issue is reproducible on 2.3-develop branch, please, add the label Reproduced on 2.3.x.
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• 5. Verify that the issue is reproducible on 2.2-develop branch.

  Details

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  - If the issue is reproducible on 2.2-develop branch, please add the label Reproduced on 2.2.x

• 6. Add label Issue: Confirmed once verification is complete.

• 7. Make sure that automatic system confirms that report has been added to the backlog.

[magento-engcom-team]

@engcom-backlog-nazar Thank you for verifying the issue. Based on the provided information internal tickets MAGETWO-97339 were created