Thanks for trying it out!
I am aware of this problem too. My decision of not showing the key combinations during the gameplay is that it will distract you realizing the melody change.
This is analogous to platformer where we do not normally show the direction, attack and jump keys on the screen because they are responsive enough and having static finger placements. Maybe it is that my idea is very original so people need sometime to get used to it (me too).the whole list of controls is too long as well as you can see in "How to Play?"