This is due to changes in the development version of mpv. Previously the range for volume was 0 - 100. Now (in the git version of mpv) the range is 0 - max amplification. So if you for example have the max. volume amplification set to 150, the range is 0 - 150. SMPlayer just adapted to that. Unfortunately now setting the volume in older versions of mpv will fail if the value is above 100.
Possible solutions:
* update mpv
* set the max. volume amplification in the smplayer preferences to 100.