It works fine here: journalctl -u service.transmission shows switching to turtle mode as scheduled.
Check timezone with command date.
Check Transmission settings in /storage/.kodi/userdata/addon_data/service.transmission/settings.json
With command "date" it seems that my system (x86 LibreELEC) does not have time syncing with Kodi time.
I set "wait for network before start" in LibreELEC setting but still can't make system's date to sync with the time in Kodi.
For now, I just change "scheduled times" in Transmission to correspond with timezone from "date" command.