takindele please read my comment here https://jira.atlassian.com/browse/JRASERVER-64415?focusedCommentId=966592&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-966592
It is really hard to say if this is a bug. The behaviour is that when importer create new custom field it puts it into project context.
When you then import to another project you cannot select the previously created custom field as it is not visible to the next project.
Workaround here is to manually change context of custom field to global after first import or create custom fields manually with global context before using any importer.
I don't think we should be messing with custom fields context from inside importer especially if the custom fields are created manually not by the importer itself.
cc: ialexeyenko maybe this bug that takindele is describing is something that Jira bugfix team wants to look at.