Thank you!
I have found the issue. There is a custom style applied on all select boxes from within the theme styles, which removes the arrow and tries to replace it with a custom icon, but it does not work properly here.
This custom CSS will reset the layout:
select.asp_nochosen.asp_noselect2 {
appearance: menulist;
background-image: unset;
border: 1px solid #dadada !important;
}
.asp_select_label {
padding-left: 0 !important;
}
After that it will look like this: https://i.imgur.com/aUx6wrM.png