Thanks for helping!
Not sure how I helped :), but glad it's working.
I don't routinely use advmame, but I did a few tests and it looks like - when mapping the input - there is a delay between pressing an input and the corresponding key being displaying in the menu. If I do a simple/single press, nothing is shown, but after the input timeout elapses, the input dialog shows the input has been acknowledged and configured. It's like the UI doesn't update immediately after 1 event processed.
We'll see if something can be done to make the input mapping more responsive.