I think it's a pleonasm because both adverbs ("mas" and "sin embargo") has an adversative meaning and adding "mas" doesn't affect the meaning of the sentence. So it's redundant.
To me, these sentences have the same meaning though using "mas sin embargo" may make the sentence more aesthetic for literature (so it will probably be more used in writing than in speaking).
Mario tiene que hacer muchas tareas, sin embargo está jugando.
Mario tiene que hacer mucha tareas, mas sin embargo está jugando.
even they are quite close in meaning to:
Mario tiene que hacer muchas tareas, pero está jugando.
Mario tiene que hacer mucha tareas, mas está jugando. (this is very formal)