I FOUND THE SOLUTION!
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There you go, it's fixed for good. pbMEPlay is just a bad method. For some reason it tends to f*** with what comes next (the battle bgm) because of unknow reasons that have to do with the trainer intro ME (which I noticed having some lag at the first time it plays). Audio.bgm_play was the one used the most in older versions of Essentials, and so far it has been way more realiable to me :)