Hi nice work. I installed transmission on my libreelec 7.0.2 on Raspberry Pi B and i see i problem that I have a lot of time ago on openelec.
The problem was that during boot, transmission starts "too early": it starts before any external disk is mounted and then it can't access to the download, incoming and watch folder that i simlinked to and external disk.
On openelec after investigations i solved by setting type to idle on the systemd service file, but it doesn't seems to work anymore: after this charge transmission daemon doesn't start anymore.
Can you please check your version with the last openelec version if something else must be changed.