All a bit strange.
I tried the same app on a Motorola Xoom a galaxy tab2, 3 & 4 using the PartialLock option.
The xoom, tab 2 & tab 4 all went to sleep and the timer operation stopped.
The tab 3 keeps going but the dimmed screen stays on.
Also wrote a little test for a galaxy s5 phone and the screen stays on in dimmed mode.