It is not reasonable, no. But “late” seems to be the new “on time” in college application season.
While it is highly unlikely FA will add to merit at this stage, If it is a close call, you will be forced to deposit at another school on May 1 and appeal the late SIR, or deposit at UC and pray for the best. This doesn’t seem right tho. Can the admissions office tell you if late SIR due to FA incomplete will routinely be approved?