Updating an IdP record even with a minor change, set Auto Redirect checkbox as true for that IdP. Now that this checkbox is read-only therefore, in order to turn it off, you always need to go to system properties and set the value of system property glide.authenticate.sso.redirect.idp as empty.

This article explains how to avoid this default update.

This is an OOB behavior and caused by a before Update type Business Rule Ensure_Fields_Changes_Tested whenever you update an IdP record.

Access Business Rule Ensure_Fields_Changes_Tested and 

Look for variable excludedFields in Script field.

Remove is_primary from above variable value and save this BR.