Sounds like you have set up your drop down as a field with local instead of global storage. The find takes you to a different current record where you previously specified "Export" and thus the value you see in the drop down changes. Use field options to change the storage option for this field to "global" and you won't see this change. Make sure that this drop down field is not the same field used to specify the category for individual records or using this drop down will also change the category for your current record when you use it.
Thanks a lot,
Now it works perfect :)