Yeah, on the rare occasion when I’ve had issues with a particular box, my usual steps are
reboot (on occasion I’ve had issues with KDE Partition Manager or calamares after a calamares crash)
I use KDE Partition Manager, being the Qt equivalent of gparted ; though if you’re familiar with gparted it’ll work too. I delete the troublesome partition(s), and create the format I do want, or whatever partition layout i want, then exit
finally restart calamares or the Lubuntu installer (now only selecting the partition(s) I setup in the prior step & opting to use (keep) them).
I’ve been using gparted a lot longer so I know it better, though I’ve found KDE Partition Manager as good (being Qt based; it’s more efficient on resources in modern Lubuntu)
Well done on solving it yourself ! (and sharing the solution so it’ll help others)