It seems there is a bug with IPv6 support. I installed CSF on an AWS Lightsail instance and in order to automatically obtain a public IPv6 address, AWS requires support for RA/SLAAC. With CSF turned on, the instance receives an IPv6 address on boot but then once valid_lft reaches 0, it falls off the interface and can't be renewed.
I have set:
IPV6 = 1
IPV6_ICMP_STRICT = 0
IPV6_SPI = 1
Operating system: AlmaLinux 9 and Ubuntu 24.04
CSF Version: v14.22
When I disable CSF, the interface is able to obtain an IPv6 address on its own again. I would expect that with ICMP_STRICT turned off that this wouldn't be an issue but it's shown to be otherwise.