Currently, it is not possible to display linked issues in the customer portal natively. There's an open feature request for that, which you can watch and vote.
I work at Valiantys Software, and we have an alternative solution with our app Exocet. It can allow you to display your linked issues on the customer portal in a data panel as show below:
See this tutorial for reference, it explains how to set Exocet up for this.
To sync comments, you can also use Exocet as presented in this tutorial.
This is not a native solution, but I hope this will help you provide your customers with visibility on linked issues.
Hey admins! I’m Dave, Principal Product Manager here at Atlassian working on our cloud platform and security products. Cloud security is a moving target. As you adopt more products, employees consta...
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