hey
welcome to the confusing realm of nested timlines
I do not know how or why or when the link between the timeline A on the canvas and the timeline A in your timeline B gets broken, but it does. (maybe its when you open timline b out of the browser) you can do various things to each timeline, anf the other will never change
its very frustrating as its not consistent
what i do, as a solution, is ALWAYS double click on my Timeline A thats in my TImeline B. this will open it up in the canvas and allow you to modify it and those mods will show in your timeline B.
The other thing you can do, which is a little more time consuming but makes the connection rock solid, is export timeline A as a native QT, Import it back in, and cut it into timeline B. each time you need to change timeline A, simply re export again, overwriting the previous export so the name stays the same, and fcp will reconnect to the file!
hope this helps
bee eph