Tracing – Legend Mystery – Mathematica Stack Exchange

I have this function:

de(a_, x_) = -1 + (E^-x x^a)/((-a + x) Gamma(a, x));

When I do

Plot(Callout(de(-7, x) // Re, "de", {10, .022}, {x, de(-7, x)} /. {x -> 4}), {x, 0, 20})

I receive the expected picture:

enter the description of the image here

However, if I change -7 at -7.5:

Plot(Callout(de(-7.5, x) // Re, "de", {10, .022}, {x, de(-7.5, x)} /. {x -> 4}), {x, 0, 20})

so I get this disappointing result:

enter the description of the image here

What is the problem here and how to fix it?