I have seen many audio exporting issues of the years with Motion. This I think will only get worse as it gets more embedded into FCPX. The solution really is to not do any audio editing in motion at all. Either export the pictures into fcpx or some editing app, and recombine with audio there. Or create a finshed audio bed and import into motion ( thats what I do)
trying to minimize edting steps so a precompose would definitely be a last step i think. i try to never use compostiros for audio editing but you'd think the basic stuff would work :/
thanks for your help!