When the Jira Cloud Migration Assistant migrates configuration data from server to cloud it omits (or leaves behind) non-standard custom field types.

Here's the list of standard custom field types that are automatically migrated with the Jira Cloud Migration Assistant:

Customers with apps that add custom fields using non-standard custom field types will need to install apps that are 'needed in cloud' and manually recreate their custom field configurations.

Example of potentially affected apps (list may not be comprehensive):

We are adding capabilities to the Cloud Migrations Assistants to migrate app data automatically. This functionality requires Marketplace Partners to build automated migration paths compatible with the Cloud Migration Assistants; they can do so using the App migration platform. The App migration platform does not support apps migrating their own non-standard custom fields.