Please consider this simple code:
Manipulate[
Plot[{a Sin[x], a Cos[x]}, {x, 0, 2 Pi},
PlotLegends -> "Expressions"], {a, 1, 2}]
Mathematica returns this output:
I believe this is a bug. Is there any workaround to cope with this problem?
I use Mathematica version 10.3.1 on Win 10 64bit. Thank you.