Clueless one here who clearly doesn't play fantasy sports
. The behavior you are experiencing is how our game has worked for longer than I've been involved with it: weekly roster deadlines do not cut off at the first game, they cut off on the day of the week in question. Changing this behavior to the first game of the week is on "The List", and I'm pretty sure it has even been worked on, but there are complicating factors related to how we get real-time data vs. overnight official data.
For the sake of clarity as well, just so nobody else gets confused, we no longer have Plus leagues. This year all leagues are free.