I tend to think they are two different things too.

To me, muscle fatigue is that my muscles cannot work. They totally malfunction. They don't feel any problem. They just don't work.

Muscle tiredness is an aching, bad feeling, hard to make it work, but it does.

For example, my right eye closes so severely without mestinon that I can't see out. It doesn't feel like anything. If it is only halfway closed, I can't tell it without a mirror.

People ask me if my eye feels tired. Well it doesn't. It feels normal. It is just not normal.