Hi Alanood,
This is an expected behavior and by design. When any editor starts editing in Default version, application creates a temporary child version of it with same state_id of Default version.When editor saves, that temporary child version does an implicit reconcile and post to Default version and Default version moved or point to new state_id. Now Default version state_id is changed.
As per your workflow, you have 25 concurrent users are editing "Default" version. Hence you find that state of the Default version is different for editors as Default is modified from saving updates from another editor and observed the below warning message:
"The version has been redefined to reference a new database state"
You can bypass this warning message by enabling autoreconciliation on the ArcMap Options dialog box.
Here are the web helps to explain "Editing Default version workflow" better:
1. http://help.arcgis.com/en/geodatabase/10.0/sdk/arcsde/concepts/versioning/editingworkflows/direct.ht...
2. http://desktop.arcgis.com/en/arcmap/10.3/manage-data/geodatabases/version-scenarios.htm
The below content copied from 2nd web help link:
Concurrent editing of the published database
Many users can edit the same version simultaneously, so the simplest way to support multiuser editing is for many editors to directly edit the DEFAULT version.
As each editor starts editing the DEFAULT version, an unnamed, temporary version is automatically created in the edit session. This temporary version is accessible only to the current editor. When the editor saves his or her work or ends the edit session, the changes recorded in the temporary version are posted to the DEFAULT version.
If another user has edited the DEFAULT version since you've started editing, when you save your work, ArcGIS automatically reconciles and posts the changes. You are notified that the version has been changed and must save again to accept the changes made by other editors. You can bypass this warning message by enabling autoreconciliation on the ArcMap Options dialog box. Whether or not you bypass this message, if there are conflicts, you must resolve them with the conflict resolution dialog box before you can successfully save your edits.
Thanks,
Biraja