Not sure if I can answer your question or tell you why this happens, I've noticed the same thing. Did you check the sketch relations and see which on is missing before and after the edit?? I don't use blocks for "Live" action, the blocks I use are static and will never change, we might add a block, but never change it once it's made - However Chris Scarlett uses blocks and edits them to make changes that are needed. You may want to check with him.
try creating a library feature out of the sketch. You should be able to drag it onto a drawing or put it into an empty drawing view from there.
I would suggest to use weldment. You can save your sketches as ".SLDLFP". Additional benefit is you can also add properties (like material, description....) to these sldlfp files and then, those properties can be used in cutlist.