Under what circumstances would a server submit new work to a miner?
The problem you are describing could be due to pool optimisation, as it is easier to prune 1 minute old shares than probably it is to look through 10 minutes of shares in order to validate them.

As for why a miner would request new work from a pool, other than as a result of longpolling or finishing a work, might be due to the work becoming invalid with time (with standard protocol a miner would request work every second). Alternatively, the miner is trying to be malicious and trying to make the pool do extra work.