If you deposit and withdraw the same amounts with no delay you diminish the value of mixing.
I am almost sure that the engine doing the mixing is not down (I expect its uptime not being related to onion, instead related to bitcoin network and some machine running 24/7 in the cloud).
It also might have checks to know when it's down so that sessions can be extended based off the downtime.
ChipMixer will also be able to trace deposits from the blockchain/their own wallets and offer withdrawals based off that after then (especially if session tokens have some relationship to deposit addresses).