The camera algorithm locks the color palette to the manual selected range of temperatures only if the temperatures range of the image includes the manual selected range (the typical use of this function responds to the need to enhance the contrast of a small part of the image).
Therefore, in your case, to have the same color/temp reference for any of the two images you have posted, the manual range you have to set should be 17.2 - 25.2 degrees centigrade.
If the max of the image is below the max of the setted range and/or the min of the image is above the min of the setted range, the camera algorithm compresses part of the color palette in order to enhance the contrast of the image. This behaviour, on the one hand is positive as it enhances the temperature differences that would otherwise not be visible, on the other hand it is negative as it does not allow comparisons between different scenarios (as you rightly observed...)
To summarize, currently, the locking of the color palette to the manually setted temperature range is only partial.
Personally I believe that this behavior responds to a specific design choice rather than a firmware bug.
For particular needs such as the one you have explained, you could ask the manufacturer to include in the menu the possibility of activating (as an option) the full lock of the color palette.
At the time, I struggled quite a bit to get the bug relating to the set humidity corrected and on several occasions I also asked to include the possibility of removing the "fusion" effect from PIP images, without any success...