Using the original tileset, it fails to draw link moving south with a magic shield. About 1 in 3 frames will be shown with him facing left; the rest of the frames are blank.
Moving South with Large Shield
Working with proposed1st.qst in build 704, I tried giving Link the magic shield on the ground at first and then in Init. Data, but couldn't reproduce your issue. There is also no bug for the mirror shield.
EDIT: I tried making a brand-new quest and doing the same thing, but still no bug.
Send a test quest.
No response in 3 days. Moving to Limbo.