@dc42 I'll rather stay at V3 codebase.
Reverting back to 2.05 requires re-setting too many things (endstops, laser control, etc..)

No, update fails:

Hi,
Updated to 3.0RC1, similar errors.
In my earlier example (peukku.nc) error have moved to another location (bottom line).
And while lasering star last line fails.
Gcode used for star:

;Project taehti
; *** START begin ***
G90
G21
M84 S0
G92 X0 Y0 Z0
;No. 1: Engraving 1
M3
G00 X15.2204 Y25.2874 F1200
G01 X10.3421 Y17.0845 F1200 S255
G01 X1.3756 Y19.1117 S255
G01 X7.3967 Y12.1317 S255
G01 X2.4490 Y3.8119 S255
G01 X11.2939 Y7.6139 S255
G01 X17.5111 Y0.4065 S255
G01 X16.6361 Y9.9102 S255
G01 X25.4043 Y13.6792 S255
G01 X16.0954 Y15.7838 S255
G01 X15.2204 Y25.2874 S255 ; this line fails to laser. 
M5
G00 X0.0000 Y0.0000
M05

More debug info:
If F value in G1 F300 is more than F1015 All lines are done as they should. If i's lower, piece is missing. And if speed is added over F1200, next line starts to suffer. Hopefully picture explains better:

Still same thing while using:
M452 C"exp.heater5" Q200 S1
Here's video:
https://www.youtube.com/watch?v=2FMmwe_Vmpk

Reload of sample and config.g.
config.g.txt
peukku.nc.txt

I did more testing.
While using attached G-Code error hits in even with S255 parameter.
First square is burned ok, then on another part start and goes well until light turns off after couple moves (somewhere around line 30-32) and comes back after next move.
Using Release 3.0beta12
Sample G-code:
[0_1573400684875_peukku.nc](Uploading 100%)

Tested with S parameter -> works like it should.
I had to ask from CAM software developer is it possible. I'm using ESTLcam at moment.
Waiting answer from there also.

Hi,
Any progress on this?
I ran in same situation with following code with Release 2.04 (Duet 2) and 1.25 (Duet 06/085).
Tested also with 3.0beta12 (2019-11-02b1) using M452 C"exp.heater3" Q200.
Config is in attached file.config.g
Seems that heater3 output is turned off.
gcode:
;Project bugbug
; *** START begin ***
G90
G21
M84 S0
G92 X0 Y0 Z0
G00 X0.0000 Y0.0000 Z5.0000

;No. 2: Engraving 1
G00 X25.7569 Y48.9645 Z5.0000
G00 Z0.5000
G01 Z0.0000 F1200
M3 S255
G01 X19.7518 Y30.6457 F750
;bug hits here. previus move is lasered ok, next 2 moves not.
;if run line by line (pasting one by one to console) all works ok.
G01 X1.1829 Y30.6942
G01 X16.1260 Y19.5851
M5
G00 Z5.0000
G00 X0.0000 Y0.0000
M05

Problem solved!
NOR gate did the trick. Although I was too lazy to visit electric shop, I used arduino to flip the signal.

int buttonPin1 = 2;
int Laseron = 8;
int buttonStatus1 = 0;
void setup() {
pinMode(Laseron, OUTPUT);
pinMode(buttonPin1, INPUT);
}
void loop(){
buttonStatus1 = digitalRead(buttonPin1);
if (buttonStatus1 == HIGH )
{ digitalWrite(Laseron, LOW);
} else { digitalWrite(Laseron, HIGH); }
}