We have done some additional testing on this and it does seem to be working correctly.
It could be a distinction between the premium and regular days. Basically if you turn on the ability to borrow from previous and future months, the system creates two time slot buckets - one of all the unused premium days for month previous, the current month, and the next month. Same for standard days.
Thus, if you have booked all your premium days for the three month period from last month to next month, you wouldn't be able to book any more premium slots until next month. However, you could still have some standard slots open which could be booked.
Let me know if that doesn't make sense or solve the problem.