contextual variable metadata (like units) is lost when writing System to file

[anandijain]

using ModelingToolkit, OrdinaryDiffEq, Unitful

@parameters t σ ρ β
@variables x(t) y(t) z(t)
x = setmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)), u"m/s")
y = setmetadata(y, Symbolics.option_to_metadata_type(Val(:unit)), u"m/s")
z = setmetadata(z, Symbolics.option_to_metadata_type(Val(:unit)), u"m/s")
D = Differential(t)

eqs = [D(x) ~ σ*(y-x),
       D(y) ~ x*(ρ-z)-y,
       D(z) ~ x*y - β*z]

sys = ODESystem(eqs)
open("lorenz_units.jl", "w") do io
    write(io, sys)
end

gives

begin
    var"##iv#264" = (@variables(t))[1]
    var"##sts#265" = (collect)(@variables(x(t), y(t), z(t)))
    var"##ps#266" = (collect)(@parameters(σ, ρ, β))
    var"##eqs#267" = [
        (Differential(t))(x) ~ σ * (y + -1x)
        (Differential(t))(y) ~ x * (ρ + -1z) + -1y
        (Differential(t))(z) ~ x * y + -1 * β * z
    ]
    var"##defs#268" = (Dict)()
    (ODESystem)(
        var"##eqs#267",
        var"##iv#264",
        var"##sts#265",
        var"##ps#266";
        defaults = var"##defs#268",
    )
end