Good news! This issue looks to have been fixed in this commit for all IPQ40xx devices:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=678569505623e50bbbbc344c7e820fb315b79ede
I have tested with SNAPSHOT r13684-3b0f698760 and no lock ups while using IPv6
I left the iperf3 IPv6 test running for 30 min with about 75Mbytes/s (one core is 100% IRQ). After installing IRQ Balance getting pretty consistent 110Mbytes/s after it juggles around the IRQ's.