Probably you should use /64 on the wan interface, not /56.
Next, (optional - just good practice) make a black hole route for your /56
/ipv6 route add dst=2a05:f080:0:100::/56 type=blackhole
Then you can just assign subnets to interfaces as you need:
/ipv6 address add interface=bridge-local address=2a05:f080:0:101::1/64
/ipv6 address add interface=bridge-guest address=2a05:f080:0:102::1/64
etc…
Be sure that the IPv6 ND settings look good for your network - by default, the ND interface “all” should be good enough, just make sure not to check managed address configuration or other configuration, unless you set up a DHCPv6 server in your LANs. (Mikrotik’s DHCPv6 server only does prefix delegation, so that won’t solve the problem)
If you’re setting up a Windows client, and you have IPv6 only on it for testing, then you’ll need to manually assign an IPv6 DNS server because Mikrotik only advertises DNS in the RA packets for SLAAC, and Windows only supports learning DNS server addresses with DHCPv6.
type=blackhole unfortunately does not exists with IPv6. Use unreachable instead.
I just noticed that dhcp-pd client automatically adds the unreachable route for the master prefix received. Kinda cool, actually. 
And yeah, blackhole would be nice to have.
Double-check your IPv6 filter rules to make sure nothing’s being blocked that shouldn’t be.
Can you:
- ping the wan interface of the Mikrotik from the Linux box?
- ping the Mikrotik’s default GW from the Linux box.
If 1 works but not 2, then I’d imagine that perhaps your WAN interface IP is configured differently than the ISP expects - double check with them what address they’re routing your /56 to, and make sure that’s the address on your WAN interface.
Something about this just seems odd if the ISP is telling you to use /56 on the WAN interface…
If they’re just dumping the entire /56 right out the interface without a specific address on your end as a routed destination, then I wonder how they expect you to break it up into LAN segments.
I just tried to ping the public address earlier in your post:
2a05:f080:0:102::254
no dice.
I can ping 2a05:f080:0:102::1 though…
Do you see 2a05:f080:0:102::1 in IPv6 Neighbors list on the Mikrotik?
Why would you need an IPv6 address on your WAN port?
You can use the whole /56 space for your address pools to be assigned to LAN clients and let the router just route.
True - and I’m the same way - I didn’t bother to put a /64 on the WAN interface, but in this particular case, it’s helpful.
From on the Internet, I can ping the wan address 2a05:f080:0:100::2
However, I cannot ping the 2a05:f080:0:102::1 address of the same router.
If there are no filter rules in the IPv6 firewall of the Mikrotik, then this leads me to believe that there’s a routing issue from the ISP to the customer equipment - i.e. it doesn’t have proper next hop information for the entire /56. It really feels like they’re dropping it off on their router’s LAN interface and saying “enjoy your /56”
Have you tried putting IPv6 DHCP-PD client on your WAN interface, and putting a prefix hint = ::/56 ?
My gut tells me that either the ISP is just putting a /56 directly on their LAN interface with you, or else they’re expecting some kind of behavior from the customer router that Mikrotik just isn’t doing.
I’d say call their customer support and try to get them to shed some light on how they expect you to break up the /56.