fix unit

Author: vyudu

src/structural_transformation/symbolics_tearing.jl

@@ -395,7 +395,7 @@ function generate_derivative_variables!(ts::TearingState, neweqs, var_eq_matchin
         dx = fullvars[dv]
         order, lv = var_order(dv, diff_to_var)
         x_t = is_discrete ? lower_shift_varname_with_unit(fullvars[dv], iv) : 
-                            Symbolics.diff2term(fullvars[dv])
+                            lower_varname_with_unit(fullvars[lv], iv, order)
 
         # Add `x_t` to the graph
         v_t = add_dd_variable!(structure, fullvars, x_t, dv)