Investigating a weird multi-head behaviour on MATE

[crazyfred]

Hello,

I updated my system from mate 1.18 to 1.22 and I'm having a weird behaviour on my triple head system.

When I log in (using either lightdm or startx) I only get 1 screen displaying the desktop while the 2 others go blank (and go in stand-by), but the desktop is still spanned on the 3 screens.

If I disable the 2 screens which go blank and then enable them, I have the 3 screens working properly.
If I don't disable the 2 screen and switch to another VT (ctrl-alt-F1) and the go back to X (ctrl-alt-F7), the display is frozen (no more graphic updates) but I can move the mouse (even if I can't do anything with the mouse).

I had a look at my log files, but I don't see any relevant message.

I tried a minimal xsession running blackbox and everything is fine.
So my primary suspect is MATE.

How could I investigate ?
How could I activate some debug messages ?

Thanks in advance.