Chocobo wont level past level 15

[Duuude007]

st 3am
Brynhildr
zhu rong
x:9.6 y:13.2 lavender beds
My chocobo was max rank level 14 so I feed it the onion and it levelled up to 15 then I took it out the stables to fight but it wasn't getting exp. Later I tried to train the chocobo but it said it was max rank. I tried to fight different level monsters but no exp was given to the chocobo. The exp bar is empty and says I have 0/2705000 exp.

Really doubtful that this is a bug. It sounds like you fed the onion too early... When you use krakkas to level it, it wont hit the next level on its own, the final 1 xp must be earned via combat. AFTER you level, you can feed an onion, to unlock progression. The 0/2705000 is what should have been the obvious cue that it only JUST fully capped, and was ready for an onion. Another cue is that the number of stars on your chocobo's menu increases by one just after an onion is properly fed.

It is highly likely that it warned you that feeding the onion would be pointless until its leveled, but you clicked yes out of habit, and fed it then anyways.

Its always been this way, sorry you got confused.