In Portfolio (Plans), I have set an issue with a value for Release. However, this value does not show up in the actual JIRA ticket in any known field - it does not show up in Fix Version(s), which is where I would expect it.
(All changes in Plans are committed/saved)
That sounds really unusual and potentially a bug... you've basically the covered the things I'd suggest to check (like making sure the changes are committed). And it should show up in the Fix Version(s) field in Jira.
One thing might be to check the other way around - if you assign the release in the fix version field in a Jira issue, is it then reflected correctly in the plan?
This might be worth raising a support issue for, also it would be good to know what version you're using ... I don't recall this being previously reported as a bug, but if you're on an early version it is possible that this was an issue subsequently fixed in a later release.