fix: handle timeseries detection for constant observed

Author: AayushSabharwal

src/systems/index_cache.jl

@@ -284,23 +284,21 @@ function IndexCache(sys::AbstractSystem)
     for eq in observed(sys)
         if symbolic_type(eq.lhs) != NotSymbolic()
             sym = eq.lhs
-            vs = vars(eq.rhs)
+            vs = vars(eq.rhs; op = Nothing)
             timeseries = TimeseriesSetType()
             if is_time_dependent(sys)
                 for v in vs
                     if (idx = get(disc_idxs, v, nothing)) !== nothing
                         push!(timeseries, idx.clock_idx)
-                    elseif haskey(unk_idxs, v)
-                        push!(timeseries, ContinuousTimeseries())
                     elseif haskey(observed_syms_to_timeseries, v)
                         union!(timeseries, observed_syms_to_timeseries[v])
                     elseif haskey(dependent_pars_to_timeseries, v)
                         union!(timeseries, dependent_pars_to_timeseries[v])
-                    elseif iscall(v) && issym(operation(v)) &&
-                           is_variable(sys, operation(v)(get_iv(sys)))
-                        push!(timeseries, ContinuousTimeseries())
                     end
                 end
+                if isempty(timeseries)
+                    push!(timeseries, ContinuousTimeseries())
+                end
             end
             ttsym = default_toterm(sym)
             rsym = renamespace(sys, sym)