I am seeing an issue when a legend is created with markers. Whenever a line includes a marker, the text in the legend corresponding to that line turns gray. I am using R2018a. The problem can be reproduced with:
% generate the signal
f = 50; % frequency
Vm = 10; % peak
phi = 45; % phase
t = [0:0.0001:3/f];
th = 2*pi*f*t;
v1 = Vm*sin(th);
v2 = Vm*sin(th - phi);
v3 = Vm*sin(th - phi*2);
% plot them
plt = Plot(t*1E3, v1, t*1E3, v2, t*1E3, v3);
plt.LineStyle = {'-', '-', '-'}; % three line styles
plt.Markers = {'o', '', 's'};
plt.LegendBox = 'on';
plt.Legend = {'\theta = 0^o', '\theta = 45^o', '\theta = 90^o'}; % legends