I have been discussing an issue on this thread in the support forum: https://forums.adobe.com/thread/2445804

I will outline the issue below again, however.

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 via the validation section of the drop down properties:

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

Here is a video clip of what is happening as well: https://vimeo.com/253833882

I am using Acrobat Pro DC, and Reader DC to test this. A user with XI has confirmed that partial drop down search works fine for him.