no, this looks unfortunate, but is not really a bug.
DiracTrace does not evaluate the trace.
But Tr[constant] gives 4*constant.
So what you are calculating is tr( tr( g_a g_b) ) which is
So, to just calculate a trace do:
This archive was generated by hypermail 2b29 : 09/22/17-08:00:01 AM Z CEST