It isn't just you and it isn't just PS3 users, I am using Windows 7 and have the same problem. The same exact hotbar 2 spots get automatically replaced with materia melding actions whether or not your hotbar is locked. It happens for me every time I learn a new ability/level synched class quest. I posted a bug report about a week ago, but so far no official response. I posted screens of what happens. Several others are also having this same problem.
http://forum.square-enix.com/ffxiv/t...-melee-actions
Not sure why it keeps happening, but
learning a new ability after being level synched seems to be the consistent issue. I am a v1.0 player and have been going back to re-do all of my class quests, so my character usually has to get level synched for those quests. When I get level synched under other conditions (dungeons, FATEs, etc) the bug does not appear. It has only been (for me) after I was level synched and (learned or re-learned) a new class ability.
I was also leveling another character on a different server (Midgardsormr) from scratch, and never saw this bug occur as I learned new abilities. That character was created in v2.0 and usually didn't require level synching when completing a class quest for a new ability. The bug seems to only affect my v1.0 character (on Sargatanas).