Obviously you didn't scroll down to the end of your frame.
BTW: Where is this vertical scrolling coming from.
@troester , I didn't scroll down because, in my test list, I have just one record. So, also me thought this strange behavior, as I didn't expect any vertical scroll but, on a fresh J!4 and F!4, with a test list, with Cassiopeia default template and Dropdown buttons, this is what I'm having.
I'm also posting an image in a list with 3 records . With 3 records, when you select the dropdown works OK and the vertical scroll bar is not displayed but, if you try to open the second or the third, there it comes the vertical scroll bar. So, it looks like the dropdown menu can't find enough space to render and, because it's rendering inside the frame, there it comes the bar.
Sorry my dummy language, but as I'm not a coder, this is the way I can express myself.
Can you please test in a new list with just one record as me?