Yea... I wrote a redmine to Jira importer. That was... interesting. I'm familiar with Redmine and its table structure. I was able to reverse engineer the json schemas (there are examples at https://support.atlassian.com/jira-cloud-administration/docs... but little in the way of actual schemas and DTOs to work from) and successfully imported everything including a number of custom fields in Redmine and comments and attachments.
The only "oops" part of the Great Import was that the users that were in Redmine but not in Jira (people who left the org and thus were never imported into Jira) had entries created as the sysadmin who ran the import.
Your case might be in a "Closed" state on the UX, but internally that can be "custom state 48083232" because some wingus decided to first start by naming it "to be closed" and then changed it to just "Closed". That can be different for every project.
This is why an import tool is a complex thing to figure out. You have to translate all the potential custom states into the "we're not Jira"'s states that make sense.