Solver can currently settle an auction after the deadline. This is against our rules (see third bullet point).
This is also problematic because orders from their solution might be added back into the next auction. Another solver might then pick up such on order and win, but they cannot execute since the order was already filled. See also this issue on the solver rewards repo.
The proposed behavior would be to denylist a solvers for settling late.
At the moment this happens relatively regularly. So initially, some grace period might be required, which should eventually be removed.