SailfishOS version Vaarainjärvi (18.104.22.168) brings the following Power key shortcuts to Jolla. Both of them affect the screen lock. It is not possible to disable these shortcut commands.
Screen lock (and the complete device lock) is explained in this article.
Immediate screen lock
Double press the Power key quickly when the display is on.
The display turns off (no change here) and the screen lock goes on immediately, regardless of the delay set for Automatic locking in menu Settings > System > Device lock.
This implies that the device lock must be entered before using the device apps the next time.
Double press the Power key quickly when the display is off.
The purpose of this feature is to make it faster to get back to using a locked device without compromising the lock utitility.