`complete(sys; flatten=false)` creates unexpected DEF parameters

[hersle]

using ModelingToolkit
@parameters a b
@variables x y
@named S = NonlinearSystem([
    x ~ a
    y ~ b
])
S = compose(S, S) # make a hierarchical system
parameters(complete(S; flatten = true)) # [a, b, S₊a, S₊b], as expected
ModelingToolkit.get_ps(complete(S; flatten = true)) # [a, b, S₊a, S₊b], as expected
parameters(complete(S; flatten = false)) # [a, b, S₊a, S₊b, DEF], but expected [a, b, S₊a, S₊b]
ModelingToolkit.get_ps(complete(S; flatten = false)) # [a, b, DEF, DEF], but expected [a, b]