You can't modify image while exporting because modifications will be exported too (in some part of file).

How to see it:
1. Work on big image (it just need to gain some time while exporting).
2. Export to png (imho: it's obvious that this is not the only bugged format).
3. While exporting make some changes on image.
4. At last we have file with part of original image and part (on bottom) after our changes maked while exporting.

Example result:
http://wstaw.org/m/2016/11/07/3.png
Yes, you see result of: export, modify, modify, end exporting.

Solution (some ways can be joined each other):
a) Block any modifications while exporting.
b) Show warning before modifications will be commited and can destroy exported file.
c) (Option in gimp's settings?) exporting can be: 1. copy whole image to other place in RAM. 2. export from that place(!). Any modifications we can make as always, because exporting is from copy of image. Copy from ram to ram is easy to make, but better way is make it as only an option and default- just block modifying while exporting.