Is it more likely for fillings in baby teeth to fall out just prior to child shedding tooth?

Depends: If the tooth resorbs on an angle, some of the retention form holding the filling may be lost leading to the filling coming out, but normally the tooth will be shed with the filling intact if the permanent tooth eruption pattern is normal.

Yes: As the permanent tooth resorbs the baby tooth root it may weaken the baby tooth crown. If this happens the filling may fall out. At that time the filling is no longer necessary as the nerve of the tooth is also resorbed.

Very possible: As adult teeth grow in, baby teeth dissolve and so it is possible that a filling in that baby tooth that is held on by mechanical retention might fall out as the baby tooth collapses. This will not happen every tine, but is possible!