When I looked at the 1000 Genomes data five samples collected in Dhaka, did not align with the others. Most Bengalis are shifted away from other South Asians because of East Asian ancestry. These five individuals, in contrast, clustered with Tamil and Telegu Dalits. Importantly, their identification numbers indicate they were sampled at the same time.
This highlights the fact that a large community of Dalits live in Bangladesh, Dhaka Dalits push for anti-discrimination law:
A considerable number migrated into what is now Bangladesh between 1835 and 1940, during a British-sponsored urbanisation plan. They worked in jobs such as road sweeping, clearing sewage, shoe repair and tea harvesting. This historical legacy of working in low-paying, difficult jobs continues today.
The genetic data suggest to me that they are indeed descendents of migrants.