IPv6 RA DNSSL lifetime is too short, not compliant with RFC 8106
This issue is almost exactly the same as issue 11105  but for the DNSSL setting.
The 'AdvDNSSLLifetime' value in the 'DNSSL' section of radvd.conf should be set to 3*MaxRtrAdvInterval (per ), otherwise the Lifetime value in the RAs is only set to MaxRtrAdvInterval (defaults to 20). On my home network with somewhat weak WiFi, these RAs were getting lost frequently enough to where Mac OS X would tell Chrome that network connectivity had changed, which would cause long-running file downloads to fail consistently.  Looks like these defaults lifetime values have been increased in radvd code , but values are still needed in radvd.conf for radvd 2.19 to override the low default values in that version.
Also, it seems like the 'Provide DNS configuration via radvd' checkbox in the UI doesn't accurately reflect the value of 'radvd-dns' in the config. When navigating to /services_router_advertisements.php, the box is always checked regardless of whether radvd-dns is currently set to 'disabled' or 'enabled'.