I only know of one reason for not being able to betray Strauss: being a Tremere and accepting the Chantry haven.
For every other character I played, it never was a problem, regardless of which version of the game I was playing.
The best order to do it is
-get the quest from Isaak
-talk to Strauss (you can even tell him, you won't betray him)
-deal with the gargoyle
-tell Isaak, but don't mention anything about Strauss or a secret yet
-return to Isaak, now you should be able to tell him about Strauss
Tuesday is Soylent Green day.