Question

Link creation and removal operations, when performed by link synchronizers, are not written to the issue history and activity streams. Why?

Answer

They are not written because doing so may affect JIRA performance. If you want them to be written, please perform the following steps:

Add a new JIRA startup system property: -Dstructure.bulkLinkProcessor.useLinkManager=true
Info
This page describes how to add JIRA startup properties.
Restart JIRA