If you want to stretch the signal longer, you will need to apply the effect to a longer selection and then trim it, rather than the other way round.
When stretching longer, the resampling interpolates new values between each successive sample value. The problem at the end of the selection is that there is no “next sample value”, so resampling interpolates between the value of the last selected sample and “nil”, which is interpreted as “zero”, hence the dive to zero at the end.
