So I heard back from Mikrotik and provide the following communications that came in last night my time.
First response: Unfortunately, in your case it seems that in the past you used a bug in your advantage. Your ISP with "RA" advertises only option called "managed-configuration". This option tells end devices - "you will need to use DHCP service". It is incorrect to install route for SLAAC in such scenario
Second response: We did dig a bit deeper and my last reply might be a bit wrong. In general it is correct, however, route still must be installed even if only "managed-configuration" option is only one included if RA lifetime is > 0. We will try to fix this as soon as possible.
So hopefully soon IPv6 default route use for Verizon FiOS and Google Fiber will be fixed.