I have checked that the problem appears with 6.18.00 as well (while I had tried with an earlier version : 6.12.06)
The following problem may be a consequence of the other bug report that I have given :
We see on the first page left, that the green curve fit is completely crazy, with Chi2FitTo.
With fitTo, it is bad as well, while less visible.
Please find attached :
-A minimum program, in the configuration made to show the problem.
-A pdf that summarise the main problem in the first page.
I have shown that to a stat forum, and experts do agree that there should be a bug in roofit. it may be a consequence of the bug of CutRange (but maybe it is a second bug of roofit).