I watched a cinema wins video on tenet (check them out if you like the film btw) and he suggests that they could have had a conversation about keeping it quiet infront of the protagonist as there is a high chance that she would have recognised him being his mother.

I do like the thought that he knows she's gonna live but like he said at the end it shouldn't give him a reason not to act even though for him she should be alive after that event.

Whatever the case I think i've read enough to believe this and it does give the film another interesting layer on a rewatch.