Multiple neighbor items.... with no IP address?

Hi to all, I would like to know if there is any reason I can find same RB reported multiple times in Neighbor list, with no IP address:

Interface: bridge1
IP address: 192.168.81.27
MAC address: ether1 MAC address
Identity: mUd1
Version: 6.19
Board name: RB433

Interface: bridge1
IP address: - empty -
MAC address: wlan1 MAC address
Identity: mUd1
Version: 6.19
Board name: RB433

Wlan1 set ad ap-bridge, WDS assigned to bridge1
bridge 1 ports = ether1, wlan1
bridge 1 stp = rstp


All other radios show only neighbor list entry (with same configuration).

I reboot RB and all goes ok.
Could be some rstp ptoblem due to different ROs version (6.19 and 6.34) on devices?
Thanks.