Classic publishing site and X-UA-Compatibility

We have a classic publishing site that uses the default master page, seattle.aspx.  In our dev tenant, the control, SharePoint:IECompatibleMetaTag, in the master page outputs a tag of:

<meta http-equiv="X-UA-Compatible" content="IE=edge">

But in the prod tenant, it outputs a tag of:

<meta http-equiv="X-UA-Compatible" content="IE=10">

Can anyone explain how the control, SharePoint:IECompatibleMetaTag, determines which IE compatibility mode to use?  Both sites are configured in the exact same way.  We’ve also checked that the prod tenant doesn’t have the https://support.office.com/en-us/article/display-a-classic-sharepoint-online-site-in-internet-explorer-10-document-mode-4b4572b7-9223-45ec-8497-557a643da12a site collection feature enabled.

There is a similar post SharePoint Online X-UA-Compatibility changes between tenants but with no solution.