tag element type (on a form) doesn't work on mobile browsers

tried advanced dropdown yes and no, tried different bootstrap classes.

it works fine on a pc browser (can typeahead and select tags), but on a mobile brower (tried safari and chrome on an iphone and chrome on android), the element shows '0 items' and when you select it, gives an empty list to choose from. can't type into field.

you can see this on https://testlocation.skyrun.com/test which is just a basic form with one element (a tag element). i put it on 'protostar' to eliminate the template as an issue.

fyi for testing, some of the many valid tags that can be selected are '1br, 3br, newone, newsite, new braunfels'