My understanding of chapter 8.6.1 is that a quantization group is a set of consecutive coding units (CU) in decoding order that have the same qPy_pred.
The smallest quantization group size is (1<<Log2MinCuQPDeltaSize) x (1<<Log2MinCuQPDeltaSize).

With the derivation described in J1003-d7, it occurs that the CUs that are inside a (1<<Log2MinCuQPDeltaSize) x (1<<Log2MinCuQPDeltaSize) block do not have the same qPy_pred. This happens when CU size is smaller than (1<<Log2MinCuQPDeltaSize) x (1<<Log2MinCuQPDeltaSize), and only 1 neighbor of the current quantization group is available, and in the same coding tree block (CTB).

An example illustrates the case the attached schematics.

The proposed modification concerns the value of qPY_prev when not set to SliceQPY :

"1. The variable qPY_PREV is derived as follows.
– If one or more of the following conditions are true, qPY_PREV is set equal to SliceQPY.
– The current quantization group is the first quantization group in a slice.
– The current quantization group is the first quantization group in a tile.
– The current quantization group is the first quantization group in a coding tree block row and tiles_or_entry_coding_sync_idc is equal to 2."

– Otherwise, qPY_PREV is set equal to the luma quantization parameter QPY of the last coding unit in previous quantization group in decoding order.