This possibly relates to [7.23.1] Invalid IPv6 prefixes after update from 7.22.3 - however, in my case I'm on the long-term branch and upgraded from 7.21.4 to 7.21.5
After this upgrade, I found that one of my IPv6 addresses was in "invalid" state:
[admin@gw2.XXXXX.net] /ipv6/address> print
Flags: I - INVALID; D - DYNAMIC; G - GLOBAL, L - LINK-LOCAL
Columns: ADDRESS, INTERFACE, VRF, ADVERTISE
# ADDRESS INTERFACE VRF ADVERTISE
0 G XXXX:XXX:XX:XX00::1/128 pppoe-out2 main no
1 G XXXX:XXX:XX:XX00::1/64 vlan256 main yes
2 I G XXXX:XXX:XX:XXfd::1/64 vlan253 main yes << THIS ONE
3 G XXXX:XXX:XX:XXff::1/64 vlan255 main no
4 G XXXX:XXX:XX:XXfa::1/126 vlan250 main no
5 G XXXX:XXX:XX:XXf9::1/64 vlan249 main yes
...
As a result it was also not created as a dynamic entry under /ipv6/nd/prefix, dhcpv6-relay didn't work, and so on.
I couldn't see why it was considered "invalid", and the problem went away simply by:
/ipv6/address
set 2 disabled=yes
set 2 disabled=no
It's working now, but I just thought it was worth a mention. I've not seen this issue before. This is an RB4011 and the relevant config sections are below.
The address is on interface vlan253. This vlan is untagged on ports ether2/ether3, but those ports are currently unplugged. However it is also tagged on ports ether4/ether5/ether8 which is where I'm using it. ether4/ether5 have vlan252 as the native (untagged) VLAN.
/interface bridge
add admin-mac=XX:XX:XX:XX:XX:XX auto-mac=no name=bridge1 port-cost-mode=short priority=0x2000 vlan-filtering=yes
/interface vlan
add interface=bridge1 name=vlan3 vlan-id=3
add interface=bridge1 name=vlan249 vlan-id=249
add interface=bridge1 name=vlan250 vlan-id=250
add interface=bridge1 name=vlan252 vlan-id=252
add interface=bridge1 name=vlan253 vlan-id=253
add interface=bridge1 name=vlan254 vlan-id=254
add interface=bridge1 name=vlan255 vlan-id=255
add interface=bridge1 name=vlan256 vlan-id=256
/interface bridge port
add bridge=bridge1 frame-types=admit-only-untagged-and-priority-tagged interface=ether2 internal-path-cost=10 path-cost=10 pvid=253
add bridge=bridge1 frame-types=admit-only-untagged-and-priority-tagged interface=ether3 internal-path-cost=10 path-cost=10 pvid=253
add bridge=bridge1 interface=ether4 internal-path-cost=10 path-cost=10 pvid=252
add bridge=bridge1 interface=ether5 internal-path-cost=10 path-cost=10 pvid=252
add bridge=bridge1 frame-types=admit-only-untagged-and-priority-tagged interface=ether6 internal-path-cost=10 path-cost=10 pvid=256
add bridge=bridge1 frame-types=admit-only-untagged-and-priority-tagged interface=ether7 internal-path-cost=10 path-cost=10 pvid=251
add bridge=bridge1 frame-types=admit-only-vlan-tagged interface=ether8 internal-path-cost=10 path-cost=10
add bridge=bridge1 interface=ether9 internal-path-cost=10 path-cost=10
add bridge=bridge1 interface=ether10 internal-path-cost=10 path-cost=10 pvid=255
/interface bridge vlan
add bridge=bridge1 tagged=bridge1,ether4,ether5,ether8,ether9,ether10 vlan-ids=251
add bridge=bridge1 tagged=bridge1,ether8,ether9,ether10 vlan-ids=252
add bridge=bridge1 tagged=bridge1,ether4,ether5,ether8 vlan-ids=253
add bridge=bridge1 tagged=bridge1,ether8,ether9,ether10 vlan-ids=254
add bridge=bridge1 tagged=bridge1,ether8,ether9 vlan-ids=255
add bridge=bridge1 tagged=bridge1,ether8,ether9,ether10 vlan-ids=256
add bridge=bridge1 tagged=bridge1,ether9 vlan-ids=250
add bridge=bridge1 tagged=bridge1,ether8 vlan-ids=249
add bridge=bridge1 tagged=bridge1,ether8,ether9,ether10 vlan-ids=2
add bridge=bridge1 tagged=bridge1,ether8,ether9,ether10 vlan-ids=3
add bridge=bridge1 tagged=bridge1,ether4,ether5 vlan-ids=248
/ipv6 address
add address=XXXX:XXX:XX:XX00::1/128 advertise=no interface=pppoe-out2 no-dad=yes
add address=XXXX:XXX:XX:XX00::1 interface=vlan256 no-dad=yes
add address=XXXX:XXX:XX:XXfd::1 interface=vlan253 no-dad=yes
add address=XXXX:XXX:XX:XXff::1 advertise=no interface=vlan255 no-dad=yes
add address=XXXX:XXX:XX:XXfa::1/126 advertise=no interface=vlan250
add address=XXXX:XXX:XX:XXf9::1 interface=vlan249
add address=YYYY:YYY:YYYY:YY89::2/126 advertise=no interface=sit1 no-dad=yes
add address=ZZZZ:ZZZ:ZZZZ:ZZZZ::1 advertise=no interface=pppoe-out2
add address=XXXX:XXX:XX:XX03::1 advertise=no interface=vlan3
add address=XXXX:XXX:XX:XX01::1 interface=wireguard1
/ipv6 dhcp-relay
add dhcp-server=XXXX:XXX:XX:XXff::53,XXXX:XXX:XX:XXff::54 interface=vlan253 link-address=XXXX:XXX:XX:XXfd::1 name=vlan253
add dhcp-server=XXXX:XXX:XX:XXff::53,XXXX:XXX:XX:XXff::54 disabled=yes interface=vlan256 link-address=XXXX:XXX:XX:XX00::1 name=\
vlan256
add dhcp-server=XXXX:XXX:XX:XXf9::64 interface=vlan3 name=jool