Mi - MI Redmi Note 5 Pro not working in warranty period

I purchased a new MI phone (IMEI- ***************, ***************) on **/**/**** but since I had another phone I did not start using it immediately. I started using it for * month. The phone worked well for a few days but then it automatically started giving me on/off problem itself many times in a single day. I thought to get it repaired during my spare time and started using the old phone again. I then recharged the phone ** days back before getting it repaired but suddenly it started behaving well. I again used it for * days but then again that old on/off problem came again. Today I went to MI Service centre (Dwarka Delhi-QDIGI code - XMIN****) to get it repaired. Service centre guys checked the phone and told me that its motherboard has to be get replaced and they have sent an approval request to change the motherboard under warranty to MI. They told that approval will come soon and they will repair the motherboard. I waited for three hours in the centre and then they told that they did not get approval. They told that I will have to pay for ****.** to get it replaced but I refused. Then they told that they will give **% discount but I refused and just give the service fees of *** Rs. There is no wear and tear on my phone. & I know that I have only used it for a few days. It's damn new. I am really surprised to know that I have to pay even in the warranty period. It’s really disappointing. I have * to * MI phones purchased by myself only and * smart TV as well but have lost confidence in MI products after this incidence. This is the first time I went to MI service centre. Had I used a phone lot then I would have understood that it could be my fault but what can happen to a phone if it is in a box. can motherboard get the problem itself? Why I should pay in the warranty period when there is no fault from my side?