When I activate the viewport in the layout previously mentioned and I check the scale it is not set to 1:150 rather it is shown as a percentage with the value of 0.003424. If I check the Properties palette and look under the heading Misc.(at the very bottom) it lists an Annotation scale of 1:50.
So at what point did you add the AEC objects? Before or after you scaled the drawings in model space?
Do you have a copy of the original drawing you received from the architect?
STOP! I may have inadvertently solved your problem.
This is what I did. I opened the drawing and activated the viewport. Next, I opened the Properties palette and went down to where it says Annotation scale and changed it from 1:50 to 1:1. I then changed the viewport scale to 1:150. I repositioned the drawing in the viewport then I locked the viewport display so that I wouldn't accidentally change the viewport scale as I popped into and out of model space via the layout. Try it and let me know if you are successful.