PDF files in temp folder are removed

Hi,
a registration form contains some pdf upload fields.
The files are well stored in the uploads/ultimatemember/temp/ folder.
Sometimes the files are then correctly moved to the user id folder, and removed from /temp/ folder.
Sometimes they are not moved to the user id folder, but just removed from /temp/ folder, so an error message appeared on the front screen : “File has been removed”, as the file is no more at the address it should be.
I don’t see the reason of this issue.
Is anybody has an idea, please ?
Regards, Sylvain