Engraving small font - the bit digs deeper and deeper as the job goes on. 6040z

[magicniner]

I cannot see this being the problem more likely the simple options, ER11 thread damaged and not locking collet properly, .

Could you speculate on how this would cause the bottom section of the Z axis motor coupling to move into contact with the screw underneath, leaving a gap in the coupling assembly which should not be there it as can be seen in the video?
:D