It's a common misconception that Bronze V is the lowest you can go.
- The MMR system appears to go from 0 to 3000.
- The starting point appears to be around 1100.
- Over 95% of the population is between 700 and 2500.
- MMR doesn't get reset between seasons.
The expected MMR for a Bronze V player is ~800. You are around 500MMR.
To put it in perspective, if we added two more leagues below Bronze (i.e.: Tin and Wood), you'd be roughly Wood I MMR.
Until your MMR catches up to your current division, you'll be seeing negative LP gains in divisions over Bronze V (there doesn't appear to be clamping in Bronze V.).