I’m sorry you are encountering a problem with export. I spent a while trying to reproduce this; I believe I finally know what is happening. There are two different bugs.
1) I store the list of images in the order you select them, but iOS reorders them (an iOS call I make apparently does not respect the order I pass in)
2) The “original file name” setting is not working reliably.
I do not have a workaround at this time for either problem. The fixes are not trivial; they will take some time to make.
I’m sorry I don’t have a better answer right now.