Currently the epic is displaying a strikethrough; however, all linked issues are not complete and the resolution is set to none. I would prefer the issue considered closed after all issues have been resolved.
You need to create your own logic. JIRA just gives you the capabilities to configure it, you still need to figure out how that would work technically with your process.
The issue is closed and crossed out because you closed it or you let it get closed out.
There is a workflow condition "Sub-Task Blocking Condition" that you can use for this; I believe it's available as part of the script-runner plug-in, but I couldn't say that with certainty as I've never built a workflow without that plug-in available. Simply add this condition too all "resolve" transitions in the workflow and select appropriate resolved states as the requirement.
Other option is to use JJUPIN plugin which lets you script workflow conditions using SIL language
Here is an example explaining exactly your case: https://confluence.kepler-rominfo.com/display/TR/Block+%60Close%60+Transition+Until+All+Subtasks+are+Closed
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