I have experienced similar problems when exporting images from LR mobile to Dropbox, with only 1 file exported when selecting multiple files. After selecting multiple images, clicking the sharing button, if you select the first "sharing" option and choose "Dropbox", only 1 file will be exported. However, if you choose "share to files" and choose the dropbox folder, all images will be exported.
I think it's a bug.
I'm using iPhone Xs with iOS 12.4.