It took me about 6 hours to find out all of this, but after reading a ton of man pages, initram scripts, and bug reports, I got a working result that takes about 2' to set up...
The point is to have a SWAP partition encrypted with LUKS, and it should be decypted during boot.
When using SysV, initram hooks and scripts in Debian worked like a charm but then, Systemd came and it's not yet fully implemented so this kind of crap happens. Systemd's cryptsetup doesn't support parameters in /etc/crypttab so using a script there is ignored:
/* Options Debian's crypttab knows we don't: