The lock scale feature locks the scale limits and also changes the color distribution in the image.

It is meant to be used when you have a narrow scale, such as when you are looking for small temperature variations on a wall, and suddenly a hot spot enters the image obscuring the slight temperature variations that you really want to see. Then you can point the camera at the wall, lock the scale and continue to see the small variations well even when the hot spot enters the image.

If you have a wide scale, the lock scale feature becomes useless because of a feature that enhances the contrast in the image. With automatically scaled images, the colors are distributed only in the temperature intervals where there is anything to see. A hot object would get about half of the colors and a cold background the other half. In between, there is nothing to see and just a few colors are used for that interval.

What would happen if you could lock the colors and turn the camera into another direction where the missing temperatures were present? Well, they would be very hard to see. In order to avoid this, the enhanced contrast is switched off when you lock the scale. That has the side effect that neither the hot object nor its background will look as nice any longer.

The same thing happens when you have an autoscale and switch over to a manual scale.