I would check (or have your Jira admin check) under "General configuration" in the Jira admin UI. There's a setting for "User email visibility" which can be toggled between:
Someone may have changed this option to "Hidden" in your case.
We also use Okta and do not have this problem. I don't imagine this would cause any issue, unless Okta is somehow wiping out the email address value on user accounts (might be worth checking to see if your users still have email addresses tied to their accounts).
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