The mount issue was ~fixed in master via nix#4289, but the changes haven’t been backported to 2.3.x.
At the time, I didn’t really anticipate how far off the next release from master would be–and other/overlapping unreleased work in master adds some friction to backporting it. (If someone wants to try, I outlined likely steps in darwin: encrypt nix volume if filevault is enabled by abathur · Pull Request #4289 · NixOS/nix · GitHub)