When using montserrat, the Dropdown list widget displays normally.

However, when I use other font, such as SourceHanSans and other custom fonts added by the user, the down arrow fails to display.

When using montserrat, the Dropdown list widget displays normally.

However, when I use other font, such as SourceHanSans and other custom fonts added by the user, the down arrow fails to display.

You should set a built in font (Montserrat …) for the indicator part of the drop-down list.