It looks indeed like a bad interaction with plymouh.
Booting with splash to lightDM = NOK Booting with no splash = OK Booting with splash, starting/stopping gdm then starting lightdm = OK
So something is done by gdm to clean the vt used by plymouth or something like that, which is not done by lightDM.
What puzzles me is that it was working at some point and I'm not too sure what changed. Here it's on Debian so it's unrelated to upstart.
It looks indeed like a bad interaction with plymouh.
Booting with splash to lightDM = NOK
Booting with no splash = OK
Booting with splash, starting/stopping gdm then starting lightdm = OK
So something is done by gdm to clean the vt used by plymouth or something like that, which is not done by lightDM.
What puzzles me is that it was working at some point and I'm not too sure what changed. Here it's on Debian so it's unrelated to upstart.