Suggestion on the MS_THUMBNAIL variable. With this value assigned [See thread subject line] then the system will create/update the thumbnail image using the lowest numbered open view (for example, if views 4-8 are open, view 4 is used) per the existing functionality, but the View is maximised and not saved as-is [as the design was last saved].
One other option could be MS_THUMBNAIL = SHEET where the last accessed sheet model/or only sheet [if only one sheet model is present in the dgn] is used as the thumbnail and the view is maximised.
Third option MS_THUMBNAIL = SHEETSAVE where the last active sheet model view is maximised and saved to the thumbnail upon exit.
Forth option MS_THUMBNAIL = BORDER where if a border is attached or placed then the thumbnail is maximised and saved to that Border attachment.
The maximised view is not permanent, it is transient, the original saved view it preserved and restored upon opening.
Further to this, in the Models Dialog box when choosing to create or modify an existing sheet model, a tick-box to create a saved view of the sheet border, and / or sheet boundary [if the boundary is enabled].
One more thing...
On the Models Dialog box which lists all the Sheet Models/Models, a toggle to assign a Sheet Model specifically as the thumbnail.
Only one Sheet Model can be assigned at a time.
So if I created a new Sheet Model and enabled it as the Thumbnail, the previously assigned Sheet Model thumbnail would be disabled in place of this new Sheet. [i.e. the thumbnail tick is reassigned to the newly created Sheet Model]
And in the Models dialog I can assign any single Sheet or Model as the thumbnail with the option to allow transient maximization of the Sheet Model view in the Thumbnail.