Patrick McHardy wrote:

 From a kernel perspective there are only complete dumps, the
filtering is done by iproute. So the fact that it shows them
when querying specifically implies there is a bug in the
iproute neighbour filter. Does it work if you omit "all"
from the ip neigh show command?

Omitting "all" gives identical results.  It is still missing entries 
when compared with the output of "arp".

Chris