Linking JP Backlog item to estimated JIRA story doesn't take estimate in JP

We have a scenario when our product owner defines high level roadmap in JIRA Portfolio.
Having backlog defined, he links already existing (estimated) JIRA issues to backlog items.
Saying "links" I mean selecting concrete JP Backlog item, then selecting "Issue link" column within it and picking "Link existing issue" option (see screenshot attached)

Story points-based estimate from JIRA issue does't get automatically uploaded to JP.
Moreover, if we sync JP and Jira Project, still empty estimate within JP (because its "arrival" from JIRA project has been waited) resets estimates in Jira Issue.

So, my question is: what should be done in order to get estimates from JIRA issue automatically uploaded to JP when exactly linking backlog item to issue, not when importing filter results to JP?