Thanks, in spite of using Tasker heavily I had not explored display off in so long that I did not realize Tasker had expanded the functionality and simply named it turn off rather than display off.
Regarding your LMT, for what this is worth I tested and compared the following which all turned off the display but with different methods and effects:
Tasker Turn off (no lock)
Tasker Turn Off (lock)
Tasker > run shell > input keyevent 26
LMT keycode 26
You could test Tasker > run shell > input keyevent 26. If that works then maybe LMT is having a permissions/root issue.
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As to the type of display-off mode being used, the following observations might not inform your situation but just in case:
For 3 of them [Tasker Turn Off (lock), Tasker > run shell > input keyevent 26,
LMT keycode 26]:
The entire display including status bar and nav bar quickly fades to black, the display immediately becomes non-responsive, and the power button will immediately turn on the display with on press.
For Tasker Turn off (no lock): A different display off mode is used in which the display is not fully turned off for several seconds. The navbar and status bar collapse as the display fades to black, then and for several seconds after the fade to black, the navbar can be expanded by swipe (over the black screen) and the screen can be woken up without power button press. A single press the power button will not turn on the display if done within the five or so seconds that the display remains dark but active; during those seconds 2 presses of the power button may be required to turn on the display. I've seen this display off mode before and I'm not a fan of it but it may not relate to your issue with the 26 Key command in LMT. Also, these graphic effects could very across devices, although I've seen them in multiple android versions and devices.