@Bule_Betawi CoSo strips all metadata from photos before even uploading. Some devices (notably iPhones) don't actually store vertical photos as "vertical", they store it horizontal and apply metadata saying "this way up". Because CoSo strips the metadata, it loses the "this way up" notice.
Make some edit to the photo (crop it even a tiny bit,) and it will re-save as "natively vertical" and post nicely.
@ehurtley thanks for the tip - it worked today.
@ehurtley thanks for the great explanation!