So I'm going to be honest, I just changed a couple of characters of the prefix just to illustrate they were different, but the suffix part is identical for some reason.
What's strange is I only see a text box for prefix ID. I'm not sure if this is why, but on the LAN page I have IPv6 configuration type set to Track Interface on WAN. Should I be using DHCP6 or SLAAC these days?
IIRC, this was a drop down list in previous versions, but here it actually specifies the valid range. The suffix is usually determined by the MAC address, so it is possible to have multiple addresses with the same suffix. However, the prefix should be different. Again, where are you seeing those addresses? At a command prompt? The interface page? The dashboard? The LAN page should say track interface and WAN.