IPv6 gateway configuration problem

Hi!

I’m trying to configure MikroTik router to work in IPv6 environment and have some strange behavior.
On the ether1 interface it is connected to switch that acts as dhcpv6-relay.
This switch is connected to L3 aggregation switch that send IPv6 RA messages with flags: ‘on-link’=1, ‘autonomous’=0, ‘managed’=1 and ‘other’=1.

When MikroTik router is connected to this switch it look like it receive RA from L3 switch and query DHCPv6-server for address and prefix, but…
1st. It sets default GW to address of first switch (dhcpv6-relay) instead of L3 switch (as it should be from RA-messages). Oddly, but first switch does not send RA-messages at all!
2nd. Network connected to ether1 is /120 (this mask is specified in RA-messages, of course), but MikroTik sets /64 instead.
As a result, no connectivity via IPv6 for hosts behind MikroTik. I have to set route to L3 switch manually.

How this can be fixed?

Forgot to mention that this behavior reproduced on 6.36.3 and 6.37.rc32.