CloudCompare forum

after merge some pc lost
if one pc is sub and it is the last in the list and the main pc is collapsed

Yes, it's a bit tricky, but if the point cloud is not in the selection (because its parent is collapsed), then the merge tool will only merge the selected clouds and ignore the child entities... It's a bit misleading. I'll add a warning so that the user is aware of the issue at least.

but in stable CC, it deletes all and keep one merged pc ( child pc is deleted and lost )
in beta CC ,it keeps all but merge selected (no things lost)

This is such a tricky UI quirk. It makes sense that CC only merges what's 'selected,' but the fact that it deletes the unselected children in the stable version is a bit brutal. Until the fix/warning is added, I'm definitely going to be double-checking my DB tree expansion before every merge.