Removing an attachment strips non-Microsoft X-* headers from a message

I've run into an odd behavior that doesn't seem to be documented. When I https://learn.microsoft.com/en-us/graph/api/attachment-delete?view=graph-rest-1.0&tabs=http from an email message via https://learn.microsoft.com/en-us/powershell/module/microsoft.graph.mail/remove-mgusermessageattachment?view=graph-powershell-1.0, Graph appears to strip all non-Microsoft X-* Internet message headers from the message.

For example, an existing X-Spam header will disappear, but X-MS-Exchange* headers will remain.

Is this behavior documented anywhere either as a bug or a feature? Is it just me?