If so, then it’s a hardware problem, most likely the HV power supply for the laser tube.
The rest of the pattern looks fine, so it’s likely a severe case of the Random Dot problem caused by the power supply allowing the tube to fire when the L input should force it off.
There are, to the best of my knowledge, no simple diagnostic tests proving that conclusion (other than the random dots themselves), so you may as well order a HV power supply and see if it improves the situation.