Total has been changed in Mathematica 12.0. The documentation seems to contradict itself. In one line it states:
Total[ $f[ e_1,e_2,...]$ ] gives the sum of the $e_i$ for any head $f$
In the next line one reads:
By default, Total only adds up elements inside List, SparseArray, StructuredArray and Association.
As a matter of fact, in Mathematica 12.0
Total[ f[a,b] ] does not evaluate to
a+b as it did in Mathematica 11.3.
Instead, the new option
AllowedHeads was introduced and one can for example write now
Total[f[a,b], AllowedHeads->Inherited] for obtaining the old output.
This is a breaking change. I wonder if this was intended by Wolfram?