Currently, we do not support full resolution export for synced in assets. Also, for local assets we do not support full resolution export from grid view. Local images are exported in full resolution only from loupe view. So can you please let us know the exact workflow you are doing like what kind of assets you are trying to export and whether from grid view/loupe view?
This is a real problem, especially with cropping. I understand the necessity for not syncing full sized raw files (although bandwidth these days is BIG), however if you are tightly cropping a large res file it should synchronize back a scaled down version w/out sacrificing the quality of the file. This is a true limiting factor of Lightroom mobile IMO.