In about a year of treating my comcast-assigned ipv6 address as static, it changed once.
Sadly, this happened despite me specifically requesting the same address as always. That caused me some grief. But it's not common.
My xfinity ipv4 changes once every few years, if that. I treat it as static and update things if or when it changes, which fortunately isn’t too much work. I never requested anything special regarding it, and I have a normal/non-business account. I wonder why some change often and others don’t?
On the other end of the connection, there are physical servers and routers. Every once in a while they change how things are connected/deployed for maintenance, upgrades, etc.