Your query looks good.
I tried it in my environment and it works as expected:
SELECT Name FROM Task WHERE Project IN (SELECT ID FROM Project WHERE SYSID="P-101")
Please check your query.
Pay attention to the following. Do you really mean empty string in C_WorksiteAddress<>'' ? I guess you actually wanted C_WorksiteAddress<>null. Empty string is not same as null.
Try to use simpler query similar to the my query for the testing. Is it works?