I'm using two SAMR21, one with BR (on A) and the other (on B) with
gnrc_networking example.
I'm on April release.
Switch on A (border router) then switch on B, I'm able to send UDP
message from Linux to 2001:db8 address of B.
If I try to send lot of data (every 100ms), it works for a while and
then B does not receive message anymore. After I see it failed, I
stopped the loop sending UDP message, I checked on BR and 2001:db8
address of B was not here anymore. I can ping B with fe80:: address on
iface 6 but can't with 2001:db8.

Why or how can a node be removed from ncache? Why the node does not
automatically reconnect with BR?

Answer from Martine:
Hi,
can you check if this is #5467?

Why or how can a node be removed from ncache? Why the node does not
automatically reconnect with BR?

As the implementer of the neighbor discovery I can say this: because
the neighbor discovery is sh** ;-). It was written in a haste and has
a lot of design flaws. I'm working on a replacement (see
#5704, happy to hear your
thoughts ;-)), but sadly I did not find some time to put heavy
implementation efforts into this yet (hopefully in autumn though, so
we have something for the October release, fingers crossed).

Thanks for reporting and kind regards,
Martine