The fields are stored in the meta section of the symbol.
That’s true for the schematic file as it is true for the symbol library file.
Dropping additional fields, besides the hard coded ones is rather arbitrary.
Footprint or datasheet is on the same level as other custom fields like manufacturer or part number or house number..
I think I have to write a bugreport/wishlist entry for this.
It makes no sense to drop a fully defined parts meta information when it’s backported into the library by artificially deciding that footprint + datasheet information is different than the other stuff.
It makes sense to scrub the reference, but all else should stay with the part when it goes into the library from the schematic.
Another point is that there is no official way to curate the complete library to find parts that deviate from the majority setting.
If I create a custom library or several of them I would want to have a system in place that allows me to get all symbols up to the same status and find the ones who deviate with their meta information.
@devbisme had written that import/export script to do this outside of KiCAD with a spreadsheet tool afai-remember.. but that should reside within KiCAD.
PS: and yes, I know you’re not the right person to lay this onto 