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 IE11OptOut site collection feature enabled.

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