Is the ABLE account considered part of the estate after the death of a beneficiary?

Yes. As part of the estate, funds in the ABLE account are subject to federal estate tax. After the death of the beneficiary, the remaining funds will be taxed and can be used to pay any outstanding qualified disability expenses of the decedent and any State claim under section 529A(f) before they are transferred to the beneficiary's estate or a successor designated beneficiary.