When I still had an official LineageOS, I was applying every weekly OTA and "su" would be preserved reliably. I never disabled encryption. Those were the days of AddonSU though, which was officially supported by LOS. Always remember that Magisk is a third-party tool before blaming the OS for it to stop working for some reason.
Today I run my own builds which I install via sideload. This requires "su" to be reinstalled at every update ... one gets used to it. At this point, I would not prefer one ROM over another just for its ability to maintain root across updates. Let's face it: us roote