Commit 2b32c485 was mostly based on the vim documentation, but it seems that actual vim behavior differs in that all deleted text spanning a line/lines are prepended to the numbered registers even when a register was selected.
It seems that I misunderstood/tested poorly what happens when an unfilled register is selected. Instead of "sparsely" filling the numbered registers, text is prepended to the numbered registers (e.g. When all numbered registers were previously untouched, the command "3dd will put the text on the register 1, not on register3)