Machines will default to the Lost and Found subgroup if they don't match any of the sorting criteria set in your system tree.
You can manually move the by just dragging them into the correct subgroup (if using ~ePO 4.5 etc) or using the actions menu to move the system. If you are moving the systems to the correct areas using these methods, you may have to disable the "system tree sorting" to get them to stay (which could cause further issues in your environment down the track depending on how your system tree is ).
Best option is to work out why the systems in lost & found aren't matching the sorting criteria of your subgroups.
Start by looking at where you think the machine should reside in the tree and review the sorting criteria. If the machines should reside in a nested subgroup, make sure the parent subgroup contains the sorting criteria of the nested subgroups. Once you are confident that the sorting criteria is correct (including any nested subgroup criteria), do a "Test Sort" on your systems in the lost & found area and move via the actions menu if correct.