If an user choose to share file from its home directory and give full access to the share, then upload option in the share URL not showing up or working.
When the shared file is a folder, upload to it are working.
what could cause this behavior?
They shared a file and not a folder. You can upload to a folder...you can't upload into a file.
Usually people share a file or a few files. Those can’t support and upload.
So you have to share a folder to support allowing the end user to upload back into it.