build.path with relative path specified creates new sub sub sub "$(build.path)/sketch/" directories on every run

MacOS IDE environment
$(ARDUINO) --pref build.path=./obj --board $(BUILD) --verify CP_Sargent.ino

After 4x invocations, the ./obj directory contains 4x nested instances of "obj/sketch". Each invocation adds yet another subdirectory structure...

./obj/sketch

./obj/sketch/CP_Sargent.ino.cpp

./obj/sketch/CP_Sargent.ino.cpp.d

./obj/sketch/CP_Sargent.ino.cpp.o

./obj/sketch/obj

./obj/sketch/obj/preproc

./obj/sketch/obj/preproc/ctags_target.cpp

./obj/sketch/obj/sketch

./obj/sketch/obj/sketch/CP_Sargent.ino.cpp

./obj/sketch/obj/sketch/obj

./obj/sketch/obj/sketch/obj/preproc

./obj/sketch/obj/sketch/obj/preproc/ctags_target.cpp

./obj/sketch/obj/sketch/obj/sketch

./obj/sketch/obj/sketch/obj/sketch/CP_Sargent.ino.cpp

./obj/sketch/obj/sketch/obj/sketch/obj

./obj/sketch/obj/sketch/obj/sketch/obj/preproc

./obj/sketch/obj/sketch/obj/sketch/obj/preproc/ctags_target.cpp

./obj/sketch/obj/sketch/obj/sketch/obj/sketch

./obj/sketch/obj/sketch/obj/sketch/obj/sketch/CP_Sargent.ino.cpp

Changing the CLI to use an absolute path is a workarround for this issue:
$(ARDUINO) --pref build.path=$(PWD)/obj --board $(BUILD) --verify CP_Sargent.ino