You can use a workaround like the following. In this example I have a /56 pool with prefix-length=60:
/ipv6 pool
add name=main-ula prefix=fdda:4a64:48c2:4500::/56 prefix-length=60
We assign a prefix from this pool to our interface (in this example ether2), the prefix is /60, which means we can no longer turn on "Advertise":
/ipv6 address
add address=::10:1:2:3:4/60 advertise=no from-pool=main-ula interface=ether2
Here the chosen prefix-id is 10, and the prefix is properly reserved from the pool and will not be used for other purposes:

Now we only have to manually advertise the prefix, but as /64 on ether2 for SLAAC for everything to work:
/ipv6 nd prefix
add interface=ether2 preferred-lifetime=30m prefix=fdda:4a64:48c2:4510::/64 valid-lifetime=1h
Of course, if your pool is dynamic (prefix coming from DHCPv6 client for example), then you'll need to do a bit of scripting (DHCPv6 client script) and do the following in the script:
- Extract address from interface, with something like
[/ipv6 address get ([find interface=ether2 global !dynamic !deprecated]->0) address].
- This is a string, so you can strip the
"/xx" part, then convert to IPv6 address with :toip6.
- Apply the
& operator with ffff:ffff:ffff:ffff:: as mask to strip the interface id part, then append "/64" to the result.
- Use the calculated value to update the
/ipv6 nd prefix entry for ether2.
With manually added entries in /ipv6 nd prefix you can also "share" that /60 prefix you get from the pool among multiple interfaces, in the example above, I can advertise and use fdda:4a64:48c2:4511::/64 for ether3 for example.
/ipv6 nd prefix
add interface=ether3 preferred-lifetime=10m prefix=fdda:4a64:48c2:4511::/64 valid-lifetime=10m
/ipv6 route
add dst-address=fdda:4a64:48c2:4511::/64 gateway=ether3 scope=10 target-scope=5
There is no need to assign the address entry to ether3! We only need to manually add the /64 route. Of course, if your pool has a dynamic prefix, then those two entries will need to be updated by the script.
As seen in this screenshot, Linux & Windows clients attached to ether2 and ether3 properly get the IPv6 and routes via SLAAC and RA: