Userform in Excel 2013 disappears while switching an active workbook behind

Hello, guys.

I have faced one problem in Excel 2013. In my project, I have a userform that contains a listbox which lists names of all opened excel books. When a user clicks on a particular name in the list, the appropriate book is activated. So the userform is always on top and while the user is clicking on the names in the list, he can see all opened workbooks getting activated one by one behind the active userform. This is the way it used to work in Excel 2010.
Now, after upgrade from 2010 to 2013, if a user clicks on a name in the list, selected workbook gets activated but the userform disappears.

The 1st question is what exactly has been broken in Excel 2013 and what is the way to bypass this problem. Could you please advise me?
The 2nd is WTF and has any monkey with its hands grown from its a$$ ever thought about inter-version compatibility?
Thank you in advance!

Excel 2013 is SDI interface and not MDI. I do not know answer to second question.

Yes, I see now. Is there any way to walk around?

I do not know. Is this form shown as modal or modeless?

the form is modal.

I think you must use Windows API like here: http://www.jkp-ads.com/Articles/keepuserformontop02.asp

OK, thank you, I will look into it...