Optional arguments/flags to pmbootstrap flasher flash_* get ignored

With pmbootstrap 3.1.0, when trying to flash to another partition, the --partition argument gets ignored:

$ pmbootstrap flasher flash_rootfs --partition USERDATA
[13:41:50] (native) flash rootfs image
[13:41:50] (native) install heimdall android-tools
Heimdall v2.1.0

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Copyright (c) 2021-2024 Henrik Grimler
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Initialising connection...
Waiting for device...
Claiming interface...
Initialising protocol...
Protocol initialisation successful.

Beginning session...

Some devices may take up to 2 minutes to respond.
Please be patient!

Session begun.

Downloading device's PIT file...
PIT file download successful.

Uploading SYSTEM

I tried to debug this and found that the partition is not getting passed to pmb.flasher.run. I tried looking in the code and it seems that that function gets called in pmb/flasher/frontend.py, which does not pass any of the optional arguments to it, so I assume this happens with all other arguments as well.