Observed variable calculated incorrectly using new discrete parameters

[bradcarman]

MWE with discrete parameter heading(t)...

using ModelingToolkit
using ModelingToolkit: t_nounits as t, D_nounits as D
using OrdinaryDiffEq
using Plots

inch = 25.4e-3
sec = 1
rate = 2*inch/sec
L = 4*inch

pars = @parameters begin
    heading(t) = 1
end

vars = @variables begin
    x(t) = 0
    dx(t) = rate
end

eqs = [
    dx ~ heading*rate
    D(x) ~ dx
]

events = [
    [x~0] => [heading ~ +1]
    [x~L] => [heading ~ -1]
]

@mtkbuild sys = ODESystem(eqs, t, vars, pars; continuous_events=events)
prob = ODEProblem(sys, [], (0, 8))
sol = solve(prob)
 
plot(sol; idxs=x)
plot(sol; idxs=dx)
plot(sol; idxs=heading)

This gives the correct result for x and heading

However, the observed variable dx plots a constant value...

As can be seen, the observed function for dx is

julia> observed(sys)
1-element Vector{Equation}:
 dx(t) ~ 0.0508heading(t)

Which means the calculation of dx is not handling the change in heading correctly.

[AayushSabharwal]

This has been fixed in the PR linked above