first I thought that there somewhere is an optin or setting to define wether or not the thumbnail will be used as it is on save or it is now, or ...
At the moment the only anser I can give is, that You should set Your view port to ISO first before saving.
You can automize that using a small macro to do so, together with a save
and as it is in life, I already have written this macro 3 years ago - supposing that it will be posted here also.
So a search should give this solution too.
Here is the macro
Simply click the exe - best will be a link to the QUAT or symbol bar or shortcut
This macro will change the view to ISO followed by a save of the document
I already did and my reseller spoke to siemens. The best way would be if solid edge maintains the current view and only saves it isometic instead of going to isometric view.