SmartThings Multipurpose sensor's battery dropped quickly

The battery readings measure voltage, which inherently fluctuates throughout the cell's lifetime. SmartThings sensors report battery life in intervals (e.g. 88% means between 88-99%); it's not uncommon for battery levels to underestimate the life of a battery at 30-35% less than what it actually is.

Batteries should last 6-12 months and until they approach 1%, there is likely nothing to worry about.