1) when Alt + click on layer, only the TOPMOST sublayer is collapsed. The rest of the sublayers remain expanded
2) The REAL bug is that when I collapse all layers, save, close, and reopen, ALL LAYERS AND SUBLAYERS ARE AUTOMATICALLY EXPANDED!!! It should remember the state of ALL layers, without regard to whether it is a sublayer or not. The USER COMMUNITY have been asking for a fix to this for at least 10 years!
Can’t confirm both of them.
Alt+collapse collapses all sublayers/containers for me (but I can agree you have to hold Alt for a while for it to work), Alt+Expand expands all sublayers/containers, and Ai remembers all expanded-collapsed states.
What version are you using?