Partial drop down search giving unexpected results

I have a drop down that is being populated dynamically: https://i.imgur.com/QKohGq5.png

Users have the ability to do a partial search of the drop down, to filter down to what they're looking to find: https://i.imgur.com/S0P3T17.png

However, when selecting one of the options from within the remaining options available, the results are not always as intended. In this case, I will search "Adult B" to choose Adult Brass Dragon. When selecting Adult Brass Dragon, the following script is enacted:

applyMonster(event.value, 0);

This should be choosing the option I chose, and passing it through my scripts to fill the rest of the form with information. Nothing else manipulates this drop down, aside from choosing the manual that needs to be chosen beforehand.

This is what has happened when I selected Adult Black Dragon: https://i.imgur.com/3q7IJ6T.png

I have searched and searched, but have not found anything regarding this issue, and it's starting to drive me a little crazy. It seems to be choosing another random option beginning with A, regardless of what I choose.