In using the Table field type with Subgroup option in a custom form, the select element which holds the name of the group is not updated by the core/misc/tabledrag.js script.
While looking through /core/modules/block/src/BlockListBuilder.php to model use from; it is the responsibility of the Block Module's 'block.js' to update the region select field. I would expect that this would be more naturally handled by tabledrag.js at it is a base feature of the table field.
As secondary feature/UX request:
BlockListBuilder does also add in a 'message' row for "...'nothing' in this region" -- which is a good helper (although the table is usable without it). Perhaps this row could be automatically generated with the table (with a property for a unique message string). Also, the hide/show of this row is left to the 'block.js' script at this time.