Related issues list: open issues not visible in most browsers

Summary

Can't see all related issues in Firefox (64.0).

Firefox:

This is not an issue in Chrome (71.0).

Chrome:

Steps to reproduce

• Create an issue, take note of the issue number
• Create a second issue and press the + button at Related issues field.
• Save the issue and see the result.

Example Project

You can check the open-source project I'm working on. The above screenshots are made here: https://gitlab.com/pleio/beheer/issues/484

What is the current bug behavior?

I can't see open issues in related issue list in Firefox

Update: also in Safari 12, issues are not visible.

What is the expected correct behavior?

I CAN see open issues in related issue list in Firefox

Relevant logs and/or screenshots

See the generated html in developer tools:

This might be related to the error shown in developer console:

The above error happens twice and there are two related issues not shown.