They're not really saying this.

The restriction is on an expense that is outside the 3-day grace period at the start or end of the bid period and is before a carryover trip. That expense cannot be slid forward into the next month. It says only back-end expenses outside the 3-day end of bid period window associated with a carry-over trip can be slid backwards to the previous month.

As I told my friend, I'm almost positive there has been such a restriction on front end carryover expense sliding forward in place for years. I remember looking up and finding guidance on carryover trip expense slides in the FOX system tutorials, so it's not something that just happened. This was years ago and I remember finding it strange that slides were allowed one way but not the other. It would just be nice to have the old guidance for reference as you said.