My app’s URL: https://nervous-tank-8406.glideapp.io/
I’ve tried to implement a math formula which adds the current value plus the previous total through a relation, but there seems to be a bug and the math column doesn’t add the previous value.

Could be the fact that it’s a lookup column. You might have to convert it to a numeric column by adding a Math column just to convert the one column. Then point your original math column to that new math column.