Performing a request to the Get avatar image by ID https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-avatars/#api-rest-api-3-universal-avatar-view-type-type-avatar-id-get can be can be accessed anonymously when performing the search for default icons. However, using and displaying custom issue type icons needs to have authentication.

With Forge applications, performing the request encounters a 403 error. Therefore, performing the same authentication for custom issue type icons as with the default ones icons would be beneficial, one possibility would be to use directly the icon URL:

Avatar appearance='square' size='small' src={type.icon}/>