Status on latest modems that work with prefix delegation

Hello, for many years I have had the DPC3939B modem at my Comcast Business site that hosts my email.  It lets me solicit a /59 prefix on my firewall so that my downstream servers/VLANs can get their subnetted /64s. This works great, and I even was able to get Comcast to add the reverse DNS for the IPv6 address of the mail server so it can properly do SMTP over IPv6. Recently we've been having a problem where the modem goes offline entirely and requires a power cycle, no amount of "refresh signals" or anything else will bring it back. The signal levels as shown on the diagnostic pages are all within spec. We have a tech coming out in a few days to look at it. I have read many accounts that the newer business modems have broken or nonexistent prefix delegation implementations. Can I get a firm answer on if this is true, and if so which ones are capable and which are not? Thanks.