Robin Williams points out there is a different issue here which can be seen by adding $< to the echo command, so the test reads:

all : A B C
all : ; @echo $@ -- $^, $<
all : D E F
A B C D E F : ; @echo $@

With that change, make-3.80 says:

A
B
C
D
E
F
all -- A B C D E F, A

But 3.81 says:

A
B
C
D
E
F
all -- A B C D E F, D

The manual says:

"$<
    The name of the first prerequisite."

Here the first prerequisite of "all" would appear to be "A".  Interestingly, if the example is reordered such that the prerequisites are all given before the command for building "all":

all : A B C
all : D E F
all : ; @echo $@ -- $^, $<
A B C D E F : ; @echo $@

Then 3.81 also says $< is "A".

Wrong order of prerequisites with 3.81/CVS

all : A B C
all : ; @echo $@ -- $^
all : D E F
A B C D E F : ; @echo $@

$ gmake381
D
E
F
A
B
C
all -- D E F A B C