I think it's a Mupad problem (or goes wrong between Mathcad and Mupad).
I was going to say: Use Mathcad 11 (with Maple) rather than 15 (with MuPad).
You can get it, sort of, to display the periodic solutions. But it still gives you only part of the full solution. Notice how the second and third solutions coincide due to the minus sign before the 3/4...
Здесь проблема не в графике, а в символьном решении. В Mathcad Prime результат такой же.
Here the problem is not in the graph, but in the symbolic solution. In Mathcad Prime the result is the same