Wrong file installed when building a machine - yocto

Question

I have 3 layers:
meta-a:
meta-a
└── recipes-my
└── mypgk
├── mypgk_1.0.bb
└── mypkg_rel
├── config.conf
└── mypkg.service
mypgk_1.0.bb:
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}_rel:"
SRC_URI = "file://config.conf mypkg.service"
meta-b:
meta-b
└── recipes-my
└── mypgk
├── mypgk_1.0.bbappend
└── mypkg
└── config.conf
mypgk_1.0.bbappend:
FILESEXTRAPATHS_prepend_${MACHINE} := "${YOCTOROOT}/meta-a/recipes-my/${PN}/${PN}_rel:"
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
SRC_URI += "file://config.conf "
I also have a meta-c layer.
meta-c depends on meta-b,
meta-b depends on meta-a.
Layers priority:
meta-a = 14
meta-b = 15
meta-c = 16
Every layer defines a machine (machinea, machineb, machinec)
When I build machinea, the config.conf file from meta-a is installed.
When I build machineb, the config.conf file from meta-b is installed.
When I build machinec, the config.conf file from meta-a is installed instead the one in meta-b which have a higher priority.
When I build machinec I want mypkg from meta-b as-is, why bitbake use the other config.conf file?
If I rename config.conf to config_b.conf (changing .bbappend accordingly) in meta-b everything works as axpected.
EDIT:
meta-c content regarding mypgk is empty, i want it to inherit from meta-b.
in meta-c layer.conf i have this:
LAYERDEPENDS_c = "b"
mypgk_1.0.bb defined in meta-a is:
FILESEXTRAPATHS_prepend_$(MACHINE) := "${THISDIR}/${PN}_rel:"
SRC_URI = "file://config.conf "
SYSTEMD_PACKAGES = "${PN}"
SYSTEMD_SERVICE_${PN} = " mypkg.service"
do_install() {
install -m 755 -d ${D}${bindir}
install -m 755 -d ${D}${sysconfdir}
install -m 644 ${WORKDIR}/config.conf ${D}${sysconfdir}/config.conf
install -d ${D}${systemd_system_unitdir}
install -m 644 ${WORKDIR}/mypkg.service ${D}${systemd_system_unitdir}/mypkg.service
}
EDIT 2:
also copying recipes-my/* from meta-b to meta-c/ doesn't work and config.conf from meta-a is installed

Answer 1

the issue was the order of direictories added to FILESEXTRAPATHS.
modifing mypgk_1.0.bbappend in meta-b to:
FILESEXTRAPATHS_append_${MACHINE} := "${YOCTOROOT}/meta-a/recipes-my/${PN}/${PN}_rel:"
FILESEXTRAPATHS_prepend_${MACHINE} := "${THISDIR}/${PN}:"
solved the issue