Tempo changes do not appear in the Event List. The reducing Tempos using the Fill Option works OK here. Experiment by reducing Tempos at the start of the Song say 120 to 10 at the end of 10 bars. Now look at the Bar spacings at the top of the main Tracks Window where you will see the actual Bar spacing change the slower the music is playing. This works at the end of of the song as well - try 120 to 5 over two bars.

It may be that the changes you are entering are two slight to be noticed - try extreme values.