It’s a limitation of the early versions of the PD spec. The PD spec now includes features that allow a device to express its needed power draw, and wanted power draw, along with info like the fact they’re a battery pack.

In addition the spec now has messages that allow a charger to renegotiate the PD contract with a device with resetting the entire USB-C connection. So if you have chargers and devices that support these elements, you can connect and disconnect devices with interrupting the power to anything else connected. The charger just does a live renegotiation and redistribution of its available power envelope.

Also means that when a phone and battery pack get plugged in, the charger can pull power allocation from the battery pack to send to the phone so the phone charges first. Then once the phone is charged, reallocate the power budget to the battery packs again.