`structural_simplify` on Mixed Difference Differential equations does not work

[ValentinKaisermayer]

ModelingToolkit.jl/test/odesystem.jl

Line 403 in 5a47dd6

# Mixed Difference Differential equations

using ModelingToolkit
@parameters t a b c d
@variables x(t) y(t)
δ = Differential(t)
D = Difference(t; dt=0.1)
eqs = [
    δ(x) ~ a*x - b*x*y,
    δ(y) ~ -c*y + d*x*y,
    D(x) ~ y
]
@named de = ODESystem(eqs,t,[x,y],[a,b,c,d])
sys = structural_simplify(de)

ERROR: LoadError: ExtraEquationsSystemException: The system is unbalanced. There are 2 highest order derivative variables and 3 equations.
More equations than variables, here are the potential extra equation(s):
 0 ~ y(t) - Difference(t; dt=0.1)(x(t))

Similar to #1166