Hm, I suspect that may be due to the fact that the field you're searching by is a Date field, and so even though the field displays as "YYYY-MM-DD", the field's data isn't actually that and so it's failing.
Could you try creating a new formula field with this:
DATETIME_FORMAT(
{Last Modified},
"YYYY-MM-DD"
)
And then getting your Zap to search by that field instead? Doing that seemed to work for me. 
