Contribution towards Interest and Principal in Flat Rate EMI during loan tenure

Hi, I have taken a loan of 50,00,000 from Shriram City Union Finance Limited at 16.5% Flat rate interest for 5 years. So they calculated the EMI as 1,52,083 [50,00,000+(5000000*.165*5)/60]. As in flat rate method they arrived at this EMI calculating the interest for the whole 5 years. My question is how the EMI is contributed towards interest and principal amount monthly. Isn't it 83,333 (50,00,000/60) principal and 68,750 (41,25,000/60) interest monthly flat throughout the tenure. I have paid 1,52,083 for 24 months since loan taken till date. According to my understanding of Flat rate method interest and principal payment in EMI remains the same the whole tenure. So for 2 years payment of EMI's I should be left with an outstanding balance of 30,00,000 with flat rate interest. But when i asked for foreclosure after 2 years payment from Shriram they said the outstanding balance is 37,35,000. I asked them, was I sanctioned loan on flat rate method and they said yes and they explained how the EMI came to that amount. I agreed but when calculating the contribution part towards the EMI they converted it to reducing balance method and came to this figure. I understand, if i paid EMI's through the whole 5 year tenure, i would end up with 26.8% interest rate in diminishing method. But when i pre-close, shouldn't i be charged in the flat rate method as sanctioned? Is it legal to convert loans from flat rate method to diminishing/ reducing balance method during the loan tenure. Due to this i am ending up paying 7,35,000 extra. They say it's normal, that's how they do it and doing it to all their customers. I am in dilemma whether to go legal. Do i have a case?