Your 'proof' stems from a misunderstanding - because you do not know local situation.
The prevailing belief in the administration at that time was that these were Buddhists in the hill tracts areas. That is what is being shown. That would be only partially right.
However these Buddhist Chakmas and Tenchangyas are itinerant migrants from Burma. They are not 'settlers' in that area and hence have no permanent domicile (since they practice 'slash and burn' agriculture (Jhoom cultivation) and move around a lot in the hills. They crossed borders since there was no border at the time, and border still is more or less open, because of the type of terrain that exists there. Out of kindness - the local Muslim rulers let these folks live there.
I will not go on a lengthy debate with you on this. You have to educate yourself about local conditions at the turn of the last century in the hill tracts and also today. In any case this is off topic.
I don't have the time to
Vast majority of tribals are loyal to BD.
Small minority of ingrates in the tribal community.