Adding a sensor breaks MTK model

General Usage
1

For some reason, adding a ForceSensor from ModelingToolkitStandardLibrary breaks the following model.

using ModelingToolkit
import ModelingToolkit: connect, t_nounits as t, D_nounits as D
using ModelingToolkitStandardLibrary
using DifferentialEquations


@named mass = ModelingToolkitStandardLibrary.Mechanical.TranslationalPosition.Mass(m=1)
@named spring = ModelingToolkitStandardLibrary.Mechanical.TranslationalPosition.Spring(k=1)
@named fix = ModelingToolkitStandardLibrary.Mechanical.TranslationalPosition.Fixed()
@named force = ModelingToolkitStandardLibrary.Mechanical.TranslationalPosition.Force()
@named fsensor = ModelingToolkitStandardLibrary.Mechanical.TranslationalPosition.ForceSensor()

eqs = Equation[
    connect(mass.flange, spring.flange_b)
    connect(fix.flange, spring.flange_a)
]

@named model = ODESystem(eqs, t; systems=[mass, spring, fix])
sys = mtkcompile(model)
initial = Dict{Num,Any}([
    mass.s => 1.
    mass.v => 0.
]
)

prob = ODEProblem(sys, initial, (0.0, 100.0)) # Works
sol_mass_nosens = solve(prob, Rodas5())

eqs = Equation[
    connect(mass.flange, spring.flange_b)
    connect(fix.flange, spring.flange_a)
    connect(fix.flange, fsensor.flange_a)
    connect(fsensor.flange_b, mass.flange)
]

@named model = ODESystem(eqs, t; systems=[mass, spring, fix, fsensor])
sys = mtkcompile(model)
initial = Dict{Num,Any}([
    mass.s => 1.
    mass.v => 0.
]
)

prob = ODEProblem(sys, initial, (0.0, 100.0)) # Fails on initialization or returns nothing depending on initial_positions
sol_mass_sens = solve(prob, Rodas5())

I figured adding a sensor should only add some “slave” equations but not over constrain the system. Is this a bug in the ModelingToolkitStandardLibrary or am I missing something ?