Hi Ive been using the plugin for 1,5 years now - awesome functionality!
One thing that have always wondered me is that when I send M81 to shutoff my heatbed as it is an external PID regulated bed, octoprint looses USB connection to the printer eventhough that isnt turned off.
Ive just enabled debug logging as I have a print running, so when thats finished Ill post the output.
But if anyone has an idea Id love to hear it 
It's possible that your printer is shutting off the connection since M81 is a default command for that. You could try using the @TPLINKOFF <ip> command instead and that should bypass any firmware specific issues here.
Edit: Also make sure that Auto Disconnect is not enabled for the specific plug you're controlling.
ok removing the M81 - kept the USB connection alive great..
but, theres "always" a but 
in my gcode script "After print job completes" I have
;Stop heatbed
@TPLINKOFF myHeatbedsIPAdress
But now the bed doesnt turn off .. as it did with
;Stop heatbed
M81 myHeatBedsIPAdress
What about if it's in your actual gcode file? Have you tried the recent idle timeout option? It kind of negates the need for gcode all together.
Ok - Ill have a look at it - I would like not to add it to the gcode file it self, but have it as an Octoprint thing
FYI: from the debug log:
Received TPLINKOFF command, attempting power off of "myHeatbedsIPAddress"
but it did not it off (yes it is a correct ip address not the txt shown )
duh! .. it was! how did I miss that! .. consider this case closed! ... Sorry!
You might want to mark my original comment the solution, not your own....
no worries, I know there are a lot of settings in there. I assumed since you didn't mention it that you had disabled that option.
Thanks for that .. its all about the eyes that see ... or not 