Stalling if not formating LUKS volumes (Kubuntu 16.10)

[glubsy]

I'm installing on LUKS encrypted volumes.
I have them already opened (cryptsetup luksOpen) and listed in /dev/mapper. They are already formated with mkfs.ext4.

Then in ubiquity, if I don't tick the "format" option, the installer will consistently stalls at the step "Saving packages" (in the bottom right hand corner). There is no more disk activity from there. I have to kill the ubiquity process.

If I restart ubiquity, and do the same steps but this time I tick "format" on every volume I plan to use (and later mount as /home, /var, etc.) then it works fine.