fix: handle edge case with MTK symbolic indexing

src/parameter_indexing.jl

@@ -507,15 +507,25 @@ function (atw::AsParameterTupleWrapper)(
     atw.getter(buffer, ts, prob, args...)
 end
 
+is_observed_getter(_) = false
+is_observed_getter(::GetParameterObserved) = true
+is_observed_getter(::GetParameterObservedNoTime) = true
+is_observed_getter(mpg::MultipleParametersGetter) = any(is_observed_getter, mpg.getters)
+
 for (t1, t2) in [
     (ArraySymbolic, Any),
     (ScalarSymbolic, Any),
     (NotSymbolic, Union{<:Tuple, <:AbstractArray})
 ]
     @eval function _getp(sys, ::NotSymbolic, ::$t1, p::$t2)
-        num_observed = count(x -> is_observed(sys, x), p)
+        # We need to do it this way because if an `ODESystem` has `p[1], p[2], p[3]` as
+        # parameters (all scalarized) then `is_observed(sys, p[2:3]) == true`. Then,
+        # `getp` errors on older MTK that doesn't support `parameter_observed`.
+        getters = getp.((sys,), p)
+        num_observed = count(is_observed_getter, getters)
+
         if num_observed == 0
-            return MultipleParametersGetter(getp.((sys,), p))
+            return MultipleParametersGetter(getters)
         else
             pofn = parameter_observed(sys, p isa Tuple ? collect(p) : p)
             if is_time_dependent(sys)