Experienced a failed copy process from an sd card (exFat). The folder was 50+ GB, contained raw images about 30-50MB each, so this bug is not limited to big files. Files App showed an empty folder on the iPad Pro 11. It also would not load the contents when viewd on the SD card.
Clean install of iPad OS, apparently, helps with that - a bit...
So, after a clean install, the folder indeed copies with its contents (yay! a truly innovative feature), but if one tries to go to a different directory during that process, the visual representation of the folder vanishes. However, the copy process continues, without any indication other than the steadily shrinking "X GB available" info at the bottom of the Files app window. The folder shows up after the copying is finished.
There's another bug here - if one disconnects the SD card during the copy process, the folder never shows up, but apparenty it is there and takes up disk space. Copying the folder again will bring up a dialogue asking what to do with a folder name conflict (choices: replace/keep both/stop)...
in this state, the iPad os is practically useless for anyone who relies on storing and copying large amounts data using iPad and external devices via Files app (like images or video files)
It appears that the major issues occur with exFAT-formatted media. Bizarre in that I have two Lightning-based uSD card readers whose associated apps have no issues with the same cards & files that Files can't handle. This isn't rocket science or new technology and Apple should have the skill base to deal with it.