There seems to be a bug in materia growth. I killed a boss that yields 100,000 AP. I equipped some triple growth items to master a bunch of materia, but it doesn't seem to be working correctly. I did several experiments, all with the same outcome.
Cloud was equipped with a Precious Watch (shown as triple growth). In it is an Osmose (170,000 AP to master). After the fight it has gained 103,392 AP (???), with 66,608 to go.
Tifa was equipped with a normal growth armor with an HP Absorb (100.000 AP to master). After the fight it has gained 34,464 AP, with 65,536 (2^16, but that may be a conincidence lol) to go. Well, at least the 34,464 is consistent with 103,392 / 3.
So my next thought was that maybe AP is shared over the three characters, so I did the same with 1 character dead. AP gained is exactly the same. So I did another test. I went to the crater and fought a dragon. He yielded 5,700 AP. I checked the Osmose materia on Cloud, which was now at 120,492 AP (so 103,392 + 3*5,700). So triple growth seems to work as expected, and AP is not shared.
Anyway, now I'm stumped. Any insights?