Hi Alisa,
Do the rasters all have the same underlying grid? In other words, do the rasters that are going into the mosaic, all have perfect cell alignment? If they don't you should expect the value to change somewhat because they will be resampled. What resampling do you have set in your environments? Also, is there any overlap between the inputs? What options were set in the Mosaic Operator parameter?
If you have support, you should open an incident so an analyst can have a look.
Best,
Eric