Hi,
I finally got a MASh network with tree stand-ins working and painting them with the placer over a hill. After reopening I can't see anymore an of the instances, I can blindly edit them with the placer but I can't really edit invisible instances. The render is still fine they are just missing in viewport.
Are you sure that you didn't change the file location files. Maya gets confused sometimes.
I just close and open the file. I didn’t change anything in between, neither in the scene nor in the location of the project. nevertheless the instances are no more visible in viewport, but render.
If ever somebody face the same problem, just hide and unhide the ass files in outliner which gets hidden when creating the mash network. By whatever reason this makes the instances visible again after reopening the scene