I'm guessing it has to do with the need to choose every moment we see on screen very carefully. Jamie telling Claire that Murtagh survived but was sent to the colonies under a bond of indenture would provoke further questions and discussion, taking up even more screen time, which is already in limited supply. I.e., there's plenty more going on that's directly relevant to whatever is driving the plot of each episode, and based on the arcs of 306-308, discussion of Murtagh's fate would have been a distraction, wherever they might have tried to insert it. (I'm inclined to consider Murtagh's survival as one big, unnecessary distraction, but that's a whole 'nother conversation that I don't feel up to just this moment.)
Based on Claire's casual mention of Murtagh in 308, I would guess that Jamie told her about his survival offscreen. Then again, Claire asking him why he went back to the prison after he had escaped – Claire should have known Jamie wouldn't have abandoned Murtagh, and him in ill health. So it could go either way, really.
Just as an aside, in VOYAGER, Claire doesn't ask Jamie directly about what happened to Murtagh; Jamie never mentioned him, so she asked Fergus, who told her that he had died at Culloden, but he knew nothing more than that (chapter 54).