Unusual finding in the deriving of the Gamma function

Hello,

In the derivation of the Gamma function, in its integral representation, you get an unusual result shown in the figure in the green rectangle. What to do to not generate it?

The result is correct, but the fact that Mathcad, in representing the generalized integral with the limit, has changed the variable of integration, assigning any, in the example X253, surprises me somewhat.

Thanks for the attention

With best regards