There are a couple of unintuitive behaviours when controlling a combo box using the keyboard.
If a combo box menu is opened using the enter button and no other interaction is made then the escape button is required to close it whilst pressing enter again does nothing. However if the arrow keys are used to highlight the currently selected/ticked item then the enter key can be used to close the menu. This is inconsistent behaviour, in both instances the selected/ticked item is unchanged. Also allowing the enter key to close the menu would make keyboard control simpler and easier to use - a user may want to quickly open a combo box to see/hear the options and having one button which controls this would be better.
Using the arrow keys to highlight items in the menu will start from the bottom/top of the menu depending on whether up/down is pressed. I think it would be more intuitive to start navigation from the selected/ticked item as in many cases a user would want an item nearby in the list. For example a user might want to change an an effect item from “Distortion 1” to “Distortion 2”, if the list of items was long it would require a lot of key presses to get to the required item and then a lot of key presses to switch it back. I think at least it would be nice to be able to choose the desired behaviour.
I’ve made suggested change here: