fix: perform toterm in Initial for Shifted variables

Author: AayushSabharwal

src/systems/abstractsystem.jl

@@ -503,7 +503,7 @@ function (f::Initial)(x)
         return x
     end
     # differential variables are default-toterm-ed
-    if iscall(x) && operation(x) isa Differential
+    if iscall(x) && operation(x) isa Union{Differential, Shift}
         x = default_toterm(x)
     end
     # don't double wrap