Background mask in LR Classic

I have a RAW image taken and edited back in 2021.   My LR version  has the latest update.  When I apply a background mask to said image, the software applies it well yet, when I try to reduce the exposure on the mask, it applies the changes globally instead of just to the mask.  Any idea what is happening?  No issues with photos imported into the catalogue recently; in recent photos the "basic" panel appears within the mask yet for the image in question, the "basic" panel appears outside of the mask.