Make won't recompile header file changes. Despite including .d dependency files - c++

Question

I have a really strange issue with the Makefile in one of my off-time projects.
I have a Makefile (shown below) that generates dependency information in .d files following https://www.gnu.org/software/make/manual/html_node/Automatic-Prerequisites.html.
The problem is that a change in one of my header files (shader.h) does not trigger the recompilation of shader.o (out of shader.cpp).
The problem started happening recently when I re-organized the directory structure in my project so I suspect it is related to that.
The strange thing is that if I do make --print-data-base it does seem like it has found the correct prerequisites matching the shader.d file.
The diretory structure is as follows:
|-src
|-engine
|-shader.h
|-shader.cpp
|-bin
|-Debug
|-OpenGLTest
|-obj
|-Debug
|-engine
|-shader.o
|-dep
|-engine
|-shader.d
Makefile:
WORKDIR = `pwd`
CC = gcc
CXX = g++
AR = ar
LD = g++
WINDRES = windres
INC = -I/usr/local/include
CFLAGS = -Wall -Werror
CXX_FLAGS = -std=c++11
RESINC =
LIBDIR = -L/usr/local/lib
LIB = -lSDL2 -lGLEW -framework OpenGL
LDFLAGS =
DEPDIR = dep
SRCDIR = src
INC_DEBUG = $(INC)
CFLAGS_DEBUG = $(CFLAGS) -g
RESINC_DEBUG = $(RESINC)
RCFLAGS_DEBUG = $(RCFLAGS)
LIBDIR_DEBUG = $(LIBDIR)
LIB_DEBUG = $(LIB)
LDFLAGS_DEBUG = $(LDFLAGS)
OBJDIR_DEBUG = obj/Debug
DEP_DEBUG =
OUT_DEBUG = bin/Debug/OpenGLTest
CXX_SRCS = $(wildcard $(SRCDIR)/*.cpp) $(wildcard $(SRCDIR)/**/*.cpp)
CXX_REL_SRCS = $(subst $(SRCDIR)/,,$(CXX_SRCS))
OBJS = $(CXX_REL_SRCS:%.cpp=%.o)
OBJ_DEBUG = $(addprefix $(OBJDIR_DEBUG)/,$(OBJS))
# ----------------------------- debug -----------------------------
clean: clean_debug clean_release
rm -rf $(DEPDIR)
before_debug:
#test -d bin/Debug || mkdir -p bin/Debug
#test -d $(OBJDIR_DEBUG) || mkdir -p $(OBJDIR_DEBUG)
#mkdir -p $(dir $(OBJ_DEBUG))
after_debug:
debug: before_debug out_debug after_debug
out_debug: before_debug $(OBJ_DEBUG) $(DEP_DEBUG)
$(LD) $(LIBDIR_DEBUG) -o $(OUT_DEBUG) $(OBJ_DEBUG) $(LDFLAGS_DEBUG) $(LIB_DEBUG)
$(OBJDIR_DEBUG)/%.o: $(SRCDIR)/%.cpp
$(CXX) $(CFLAGS_DEBUG) $(CXX_FLAGS) $(INC_DEBUG) -c $< -o $#
clean_debug:
rm -f $(OBJ_DEBUG) $(OUT_DEBUG)
rm -rf bin/Debug
rm -rf $(OBJDIR_DEBUG)
# ----------------------------- dependencies -----------------------------
# Generate dependencies in *.d files
$(DEPDIR)/%.d: $(SRCDIR)/%.cpp
#test -d $(DEPDIR) || mkdir -p $(DEPDIR)
#mkdir -p $(dir $#)
#set -e; rm -f $#; \
$(CXX) -MM $(CFLAGS) $(CXX_FLAGS) $(INC) $< > $#.$$$$; \
sed 's,\(.*\)\.o[ :]*,$(OBJDIR_RELEASE)/\1.o $(OBJDIR_DEBUG)/\1.o $# : ,g' < $#.$$$$ > $#; \
rm -f $#.$$$$
# Include the *.d files
include $(patsubst %,$(DEPDIR)/%.d,$(basename $(CXX_REL_SRCS)))
# ----------------------------- targets -----------------------------
.PHONY: before_debug after_debug clean_debug
all: debug
shader.d:
obj/Debug/shader.o dep/engine/shader.d : src/engine/shader.cpp src/engine/shader.h \
src/engine/transform.h src/engine/camera.h src/engine/constants.h

Answer 1

I'll answer this since the issue has been found by #G.M. in the comments.
Turns out that the dependency generation was flawed.
In shader.d obj/Debug/shader.o should really be obj/Debug/engine/shader.o. Modifying the sed command as shown below fixes this.
sed 's,\(.*\)\.o[ :]*,$(OBJDIR_RELEASE)/$(subst $(SRCDIR)/,,$(dir $<))\1.o $(OBJDIR_DEBUG)/$(subst $(SRCDIR)/,,$(dir $<))\1.o $# : ,g' < $#.$$$$ > $#;