It's possible that the component responsible for outputting the received Bluetooth audio signal to the instrument's speaker and headphones has developed a fault.
I wonder if the Line In audio connector has the same behaviour?
Either way, it's probably best to seek technical support assistance from the retailer that sold the instrument to you and/or the Roland distributor/subsidiary in your country.
Thanks for your response, unfortunately I bought it 2 years ago in the US and there's no official Roland distributor where I live (or there could be, I'm thinking of a prestigious name now). I still believe that it's not an internal fault, as the problem was first found on my iPad then a few weeks later, my iPhone (which means while streaming BT audio by my iPad only output mono audio, pairing my iPhone worked perfectly). I will try to see if this is true by pairing a brand new device tonight...
Anyway, what does the Line In do? Sorry, I'm not familiar with audio terms. But other functions which involve the speaker, apart from streaming BT audio, work just fine (e.g. playing internal demos or recording a performance)