It's a feature of that particular font design. There is no distinction between a horizontal and slanted hyphen. Some fonts such as Adobe Garamond, Monotype Goudy, Goudy Old Style (URW), have a slanted hyphen, but the vast majority of serif fonts don't. Some also have a hyphen that looks more like a tilde, but again this is just a design choice.
The reason ...
Use the Direct Selection Tool to select all the parts outside the
area to fix
Menu Edit > Cut
Use the Join Tool to join the open path following the character shape
Past in Front the cut shape and fix the open paths
You can add the 'ABC - Extended' input sources in the system preferences. Then you can type the Pinyin character easy with 'ABC - Extended'.
First type option + ` and then type o, you will get the ò.
option + a then type a => ā
option + e then type a => á
option + v then type a => ǎ
option + ` then type a => à
option + e then type v => ǘ