multicast msdp anycast topology

Hi,

I have the following topology:

   R1 ------ R4
    |             |
 ------          | 
 |     |          |  
R2   R3       R5--R6

R1 & R4 has MSDP configuration between it (anycast IP: 55.55.55.55) and each router has its respective loopback (R1: 1.1.1.1/32, etc.). On R6 on loopback I join-group 239.0.0.1. Now when I ping from R3 I can see that initially everything is working correct:

R3#ping 239.0.0.1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.0.0.1, timeout is 2 seconds:
Reply to request 0 from 6.6.6.6, 260 ms
Reply to request 0 from 6.6.6.6, 304 ms

but then almost immediately afterwards pings stop working:
R3#ping 239.0.0.1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.0.0.1, timeout is 2 seconds:
.

after I wait some time (for sure 3min, till mroute entries fully expires) situation repeats (i.e. again I can see two replies and then no answer at all). While checking mroute on R3 I can see (3.3.3.3 is its loopback, 100.0.0.3 is LAN interface):

(*, 239.0.0.1), 00:05:31/stopped, RP 55.55.55.55, flags: SPF
  Incoming interface: FastEthernet1/0, RPF nbr 100.0.0.1
  Outgoing interface list: Null
(3.3.3.3, 239.0.0.1), 00:00:47/00:02:49, flags: PFT
  Incoming interface: Loopback0, RPF nbr 0.0.0.0
  Outgoing interface list: Null
(100.0.0.3, 239.0.0.1), 00:00:47/00:02:49, flags: PFT
  Incoming interface: FastEthernet1/0, RPF nbr 0.0.0.0
  Outgoing interface list: Null

after some time I can see entries like:
(*, 239.0.0.1), 00:08:24/stopped, RP 55.55.55.55, flags: SPF
  Incoming interface: FastEthernet1/0, RPF nbr 100.0.0.1
  Outgoing interface list: Null
(3.3.3.3, 239.0.0.1), 00:03:40/00:00:55, flags: PFT
  Incoming interface: Loopback0, RPF nbr 0.0.0.0, Registering         <----
  Outgoing interface list: Null
(100.0.0.3, 239.0.0.1), 00:03:40/00:00:55, flags: PFT
  Incoming interface: FastEthernet1/0, RPF nbr 0.0.0.0, Registering          <----
  Outgoing interface list: Null

Can you please help and advise:

1) what could cause such strange behavior (network topology is stable) ?
2) what means 'registering' after ~2min in show ip mroute entries ?
3) is it expected to see OIL as null ('Outgoing interface list: Null') just after successful sending packet to the receiver ?

Thanks a lot, Marc

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