I created the bar using the accent in the left side menu, then selected gamma from the greek alphabet.

The code provided does not output the desired symbol with a bar above, unfortunately. Do I have to modify it somehow?

By the way, other accents seem to work with gamma.

If I use your exact term, I end up with (a+b+c)/a as entered. But I had to paste /a from clipboard for not ending up with a fraction if that makes any difference.

So, (a+b+c)/a is not changed. However, (a+c+b)/a and similar combinations are being changed to (a+b+c)/a, so it looks like sorting to me.

My term contains both roman and greek letters, but surely there must be a way to disable or circumvent sorting.

Maybe this is of any use:

plot(x, x = 1 .. 5, caption = typeset((-a[IA(WL)]*`#mover(mi("γ",fontstyle = "normal"),mo("ˆ"))`[WL]+lambda-p+theta)/lambda))