The exchange should be the one to bear the losses. In a situation where the exchange is going to halt all activities, they have all the data of users who are currently opening trading positions. The exchange might use the last data before completely stopping operations. However, decisions like this may be outside the exchange's plan. So there is a possibility that the exchange will also take their own policies to manage users who are still opening trading positions when a hack forces the site to be temporarily shut down.
In crypto exchanges it's never been easy for them to cover Binance without any doubt, one of the top exchanges that has enough sources and their assets to cover, but it's not easy for many others just because of this; it always depends.
If any exchange has enough, they surely cover it, but in case they do not have enough, they will share, or they will shut down their business and have no future because few exchanges already face issues like this, and now they are out of business.
Hacking has never been in any policy, but still it exists and has done damage, which is a big problem for exchanges and users. Just because of this, most things have never stayed on the same level.