Happened on the other pc too. Well I know now that it only happens to grouped archs. Only grouped archs that have that have the first group If you ungroup them or cut stuff out they are save.
I started getting this recently, with the message about orphaned visgroup. It didnt make my objects actually "disappear" itjust hid them in a group names _orphaned objects so I could get them back by unhiding the visgroup.
Long story short: Windowns reinstalled Steam and stuff reinstalled (cause hammer bitches without regristry entrys) Open the vmf of my map everything is twice as big as it was bevor :/ ?!!?!?! if that wouldnt mean that 50% of my map is now beyond the grid i could select it and resize it. FFS BUGGY SHITTY HAMMER