This is weird, I assume it is to do with the magnets stuck under the heater plate.
I spent time getting my Z leveled to paper thin gap. Then I had the 9 point level fail as the bed hit the belt screw (Y). So I moved the bed over towards the home sensor. Next Z home and the hot end dug deep into the bed mat, deep channels cut. That sheet is toast. So I’ve readjusted Z home again...
you ever get this figured out?
if the hotend contacts the the platform during Z axis Home. its likely the probe needs to be lowered. so it can trigger earlier. preventing the crash.
i had a similar incident. it was during a print restart. my printer froze during printing. i cycled the power. when it powered back on i touched resume print and the hot end went out away from the print lowered, then drug across the platform leaving a 3' or so gouge in the plastic of the flex plate. i lightly sanded it down with 220 grit sandpaper. i've printed over that spot and it still works. just sand the protrusions of the edges of the gouge down flat. lightly running your finger across it you should still feel the gouge but not a lip
Moved past this now, but there are a few issues here.
1) The Y bed position is tight for DHE, the 9 point is set to auto size based on the bed size, so it moves the hot end off the table for the points along Y0, this can hit the Y belt screw. Haven't looked into changing this in the Marlin yet.
2) make sure you don't put a magnet under the spot where the Z home defaults too, will make extra metal and the sensor too high at that spot and can cause collisions with bed elsewhere as the sensor doesn't see the extra metal..
Never tried Resume print.
For me it does hit the belt screw, too, although for me it is the X axis (platform) belt screw. You are right, the dimensions are really tight for the dual hotend. Maybe the Z home position magnet also is a problem for me, didn't think about that. Thanks!
The auto leveling does make strange things. I do think of two sources for this. First the warping of the printbed, which is now officially an issue. Second the autolevel values don't seem to be saved to EEPROM and so the get lost when rebooting. One can activate this feature inside Marlin but I fear this overwrites something that shouldn't be overwritten.
so why is my bed short when doing the self leveling