Same subnet in radvd.conf with two different prefix lengths

[tbclark3]

I am getting a /56 from Comcast on WAN, and have LAN configured to track WAN.  It appears to me that Comcast is actually providing a /59 because the LAN address is a /59 even though I requested a /64.  This causes radvd to advertise the correct subnet but with a /59 length.  Many Linux clients won't accept a /59, so I added the same subnet on the RA services configuration page with a /64 length.  The /64 got added to radvd.conf in addition to the /59, resulting in the following:

interface wlp3s0
{
AdvSendAdvert on;

Note: {Min,Max}RtrAdvInterval cannot be obtained with radvdump

AdvManagedFlag off;
AdvOtherConfigFlag off;
AdvReachableTime 0;
AdvRetransTimer 0;
AdvCurHopLimit 64;
AdvDefaultLifetime 60;
AdvHomeAgentFlag off;
AdvDefaultPreference medium;
AdvLinkMTU 1500;
AdvSourceLLAddress on;

prefix 2603:3023:31b:dbc0::/59
{
AdvValidLifetime 86400;
AdvPreferredLifetime 14400;
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr on;
}; # End of prefix definition

prefix 2603:3023:31b:dbc0::/64
{
AdvValidLifetime 86400;
AdvPreferredLifetime 14400;
AdvOnLink on;
AdvAutonomous on;
AdvRouterAddr on;
}; # End of prefix definition

route ::/0
{
AdvRoutePreference medium;
AdvRouteLifetime 60;
}; # End of route definition

RDNSS 2603:3023:31b:dbc0:225:90ff:fe87:21f9 2603:3023:31b:dbc0:ec4:7aff:fe97:e468
{
AdvRDNSSLifetime 20;
}; # End of RDNSS definition

DNSSL ferree-clark.org ferree-clark.org
{
AdvDNSSLLifetime 20;
}; # End of DNSSL definition

My understanding is that radvd shouldn't advertise anything less than a /64–and, in fact, it is causing a string of errors:
May 17 17:41:17 radvd 28044 prefix length should be 64 for igb1

I am working around it by running a cron job once a minute to correct the radvd.conf file if it is incorrect.

There must be a way to configure radvd to correctly advertise a /64 even though Comcast is giving us a /59, but I can't figure out what it is.

Can anybody help?