Splitting མངས་བས་ wrong?

[lothelanor]

མངས་བས་ should be split as: མངས་བ/n.v.past + ས་/case.agn (POS added for illustration),
but botok somehow splits it as མང + ས་བ + ས་ which seems odd? ས་བ exists as a noun of course, but the other case seems more common?

Similarly with དམ་བཅས་པ་