Your snake plant has root rot. Even when you allow the surface of the soil to dry before watering, have you ever considered what is happening down where the roots are near the bottom of the pot? Sanseveria will last pretty much forever, if you let them get completely dry all the way to the bottom of the pot between waterings. Testing the soil moisture will do that for you: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
The good news is that these plants are so strong that you might well save it. Pull yours out of the pot, remove the soft leaves, cut off all the brown and/or mushy roots, gently wash off the old soil, and repot in well-drained mix (I like to use 50-50 cactus mix and perlite) This article has more information: https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm.