Does TimeMachine has an higher priority than the sleep delay? When a long backup will soon occur or is running, will it go on and end even if the enter-sleep-mode settings is foreseen earlier?
There is a sleep assertion for Time Machine. So while doing a backup your mac won't enter sleep mode. If you are not sure, while a backup is in progress, check with this command: pmset -g assertions. In the processes that prevent sleep mode you will found: BackgroundTask named: "Time Machine backup"