Jira Data Center 9.4.7
We have someone leaving the company and so I'm in the process of transferring ownership of his various Jira artifacts (Boards, Dashboards, Filters, Projects, Automation Rules)
For one of his Dashboards I'm getting this error when I try to transfer ownership:

I can't edit this Dashboard's shares to figure out how to remove that non-existent user (I think this has come from legacy where the user used to exist in one or more historical Jira User Directories but they no longer exist anymore)
Any ideas how I can get around this error and then successfully transfer ownership?