Reindexing of old v.6 index keeps stopping with reason kibana was restarted

Hi,

we're preparing to upgrade from v.7.10.0, the elk stack is running on an Ubuntu cluster with 3 nodes, although we have an index which we have to prepare for the upgrade, it keeps on looping:

Details
[some]-alias

This index was created prior to 7.0 -
index requires conversion to 7.x format


Reindex [some]-alias
Index is unable to ingest, update, or delete documents while reindexing

If you can’t stop document updates or need to reindex into a new cluster, consider using a different upgrade strategy.

Reindexing will continue in the background, but if Kibana shuts down or restarts you will need to return to this page to resume reindexing.
Reindexing process

Setting old index to read-only

Creating new index

Reindexing documents
4.

Swapping original index with alias

=-----------

Reindexing process

Setting old index to read-only

Creating new index

Reindexing documents
This step was paused due to a Kibana restart. Click 'Resume' below to continue.
4.

Swapping original index with alias

===========

How can we fix this?

Thanks.