In my experience it is an unbound convention that flycutting tips run on center. Endmills cut on the apex, lathes run on the apex; so I think it was just a case of copycat to deem flycutting be so. BUT, endmills and lathe tools benefit from helical or mechanical equivalent top rake to generate a chip. Many forget to provide that for flycutters. And lathes will still cut below center, above not so much! Top rake isn't built into brazed tools.
The centered cut works as expected, the variation works better; less chatter at higher feed and reduced noise from the machine head.
Long ago, we needed aluminum sections about 7" wide, 2" thick, and a 4" wide trough down the middle. It wound up .8 deep, with a .3 radiused corners. At 10" long and required surface finish, bandsawing was out. I made two solid, brazed carbide flycutters, with integral R8 shanks. Basically identical, the first was .100 under diameter and .05 larger radius, second on size for width and radius. #1 ran .500 deep in one pass. #2 ran .8 deep, then retracted [up] to blend the undercut. The seat [1 per cutter] for carbide blank was .7" below center. Those were roughly profiled form tools, silver soldered in place. Then I used just a overgrown spindex to create the diameter accurately, based off the spindex centerline. Radii were freeform ground, and blended by hand.
I don't recall what RPM or feed rate they ran at, but the quarter sized chips flew over 10 feet.