In an effort to be more helpful than the "expert" who offered two year old advice and didn't bother to read your post...
Go to Resource Setup and enable to new timesheet experience in there. It is apparently reset during the upgrade, when the option moved from Feature Management.
It's bugged me all morning, but finally tracked it down going through the code (and I am not a very good programmer, so it was a bit of a slog).
Hope that helps. Have a great day.
