"Lost" mass is not really lost in that sense. It is converted into energy - light, motion, potential, or the like. It stays inside the Universe, be it open or closed.
That was what I was saying in one sense, if the universe is a closed system. Nothing can really be lost, per se, in a truly closed system. Merely lost to observational limits.
There are however theories which posit that there are wormholes, or objects, black holes, etc. which allow transits or pathways to other universes (or dimensions.)
That goes well beyond the original question in part, but is germane to the idea of whether the universe is actually a closed or open system, and so can anything be truly lost, or even gained.
Of course then one has to define what exactly is a "universe," and where do the frontiers and limitations of a universe actually start and end?
If you define a universe as a given set of particular traits and forces, then it is self-contained, regardless of whether it is truly closed, or not, and can be differentiated between other universes, regardless of whether those universes can effectively interact.
If you define a universe as the
summa esse (omnia), or suma in toto of all that can exist, does exist, or may exist, then there is in effect only one universe. (Though it still doesn't say it is necessarily closed as it may be infinite.)
But if universes are separate things, and can interact, then it is possible to "lose" things between them (matter, forces, energy, mass, etc). Though loses may equate through such interactions.
Of course that still remains both a theoretical and a definitional consideration.