Startup Disk Creator doesn't create an extra `writable` partition as it used to

Something strange.

2 days ago, I used Startup Disk Creator (SDC) to prepare a USB Drive for an Ubuntu 24.04.1 install (on my 22.04). The USB stick is a USB-C (USB 3) 64GB drive.

That created the usual bootable Ubuntu install partition + another ext4 partition named writable using the remaining space, that's convenient to move data using that extra partition.

Today I used again SDC on a USB-A (USB 3) 32 GB drive (still on my 22.04). But this time it didn't use the remaining space (~23GB) to create a writable partition. The 23 GB is lost.

So I thought it's maybe because 32 GB is too "small", or maybe because it's USB-A...

To be sure, I erased the initial 64 GB USB-C drive (dd...) and run SDC again ...

This time it didn't create the writable partition anymore...

Are there other criteria to consider (or it's random??)

-- edit --

1. I realize that maybe (don't remember) I only mounted the USB drive on the newly installed 24.04.1 system (thanks to that drive), and 24.04 may show a partition that 22.04 doesn't?

2. Or, another possibility, the writable partition is created on the USB drive during the install?

Can someone please confirm 1. or 2. (or ...)