Hi !
Sorry for the picture instead of code, but when copying / pasting code from my notebook, strange characters appear instead of Mathematica input code :
\!\(\*
TagBox[
FractionBox[
SuperscriptBox["E",
RowBox[{"-",
FractionBox[
RowBox[{"t", " ",
RowBox[{"(",
RowBox[{"tau", "+",
RowBox[{"r", " ", "tau"}]}], ")"}]}],
RowBox[{"2", " ", "Q", " ",
SuperscriptBox["tau", "2"]}]]}]], "p1"],
HoldForm]\)
Concerning my issue, I found the solution : in the second case to obtain the proper simplifcation, I have to apply ReleaseHold[ ] function on the expression before the Replace[ ] invocation.
Cheers!