The attached makefile demonstrates the problem.

I think the following discussion may be related.

Make manual section 10.5.4  (How patterns match)
is ambiguous on which "directory names" are reattached
to a prerequisite after a rule is selected.

Given:

%.o %.d: %.c
<tab> $(COMPILE.c) ... generate .o and .d

../c1.o ../c1.d: src/c1.c

c1 should match the pattern rule above.
The question is which directory name is reattached, I.e.
the one from the target or the one specified with the prereq.

../c1.o ../c1.d: ../c1.c
OR
../c1.o ../c1.d: src/c1.c

If a prereq has a directory specified, we would always
want to use it in preference to the directory of the target.
Is this what make does?