Was told I can have static v4, or v6, not both

Right now I have a small range of static IPv4 addresses from Comcast. I've been using these for years. However, I want to enable IPv6 on a single server, to start migrating it to full dual stack support (both v4 and v6), anticipating the day when it will have only v6.

The person on the phone indicated I can have either static v4 address block, or a static v6 prefix, but not both at the same time - that Comcast would not provision both to be enabled simultaneously.

Is that truly the case?! I just don't feel comfortable using a dynamic address for v6, because pointing a DNS record to a dynamic address, trying to get a reverse entry set up for my mail server, etc. - that just is crazy to do with a dynamic address.

As a result, I feel I am stuck in IPv4-only land for my server - which is really, really odd. I would think I should be able to set up dual-stack on my server, if I choose to.

In particular, here is what DOES work:

a) I use the EUI-64 address on my server, using the dynamic /64 IPv6 prefix I learn from the Comcast gateway. For example, my /64 prefix from the gateway is: [2601:647:wxyz:a400] (I am hiding a portion of the prefix as 'wxyz' in this email). My EUI-64 address is: [2601:647:wxyz:a400:201:c0ff:fe0d:753e]. This works fine. I can ping6 from this, ping6 to this, and I can even serve up web pages from it via Apache HTTP. 

b) I pick an address of my own from the /64 prefix from the gateway such as: [2601:647:wxyz:a400:10::1] and use that. I can ping6 from this, ping6 to this, and I can even serve up web pages from it via Apache HTTP.

What does NOT work:

c) I pick an address of my own within the static prefix range listed on my account page, which is [2603:3024:WXYZ:4600::/56] (again, I am hiding 16 bits of the address as 'WXYZ'). Traceroute6's to it show that packets reach a Comcast router within 30 miles of my home, but don't get any closer. (The traceroute6's for cases (a) and (b) go through the same 30-mile-away router, but stop there).

Is there no way to get a static IPv6 address from Comcast, at the same time retaining a static v4 address?