So I tried with few ISPs. 
ISP 1 - Asianet -  PPPoE - 200 mbps  plan. I tried with the cloned MAC, but the download speed was throttled to 50M. The upload speed shooted upto 200M. Since I knew the LCO, i asked him to release the old MAC and tried with the XX530 MAC. There was no throttling, i was getting the same speeds that i paid for.

ISP 2, Asianet - IPoE - 200 mbps plan. There was no problem after cloning the MAC. I was getting the assigned speeds i. e. 200M.

ISP 3, KeralaVision - PPPoE - 200 mbps plan. Had to clone the MAC address of the original NETLINK router, then the download speeds capped to 50M. There was no problem with the upload speeds. The LCO didn't even bother to release the mac address. So i stopped the testing there.

Hence I got a conclusion that if the connection is pppoe and if we need to use the mac address other than tp link's (cloning), the download speeds gets throttled to 50M.

@shu006  Hey, I sent you a direct message can you check it and get back to me.

  @shu006 Oh that's nice. 
I had a talk with TP-Link's engineers today and after changing some settings (just changed DNS Servers 🙄) and running multiple speed tests he concluded that the ISP is throttling the speed due to 3rd party ONT. It could be true, but I still not convinced.
I'd like to run some tests with another 3rd party ONT from V-SOL so I have concrete proof that its a TP-Link firmware issue, if it really is that. But I'd have to shell out another ₹3000 for that ONT.
You could help me out a bit, so maybe I wouldn't have to buy it. If RailTel doesn't bind MAC at all you can try running 2 speed tests-

1. One with MAC Cloning Disabled
2. One with MAC Cloning Enabled (put any Random MAC address). 

If with MAC Cloning download speeds are again being limited to 50Mbps and without MAC Cloning speeds are back to normal, that would be great proof that it's really a firmware issue.
I could guide you through it if you're up for it and need help. We can gather Screenshots and Screen Recordings as proof.
It would be of great help for all of us having this issue and TP-Link would at least look into it more seriously.

Also there is no issue with using blue cable with green port. 

Yeah, there won't be any issue per say. It'll work perfectly normal. You would just get lower RX power, but won't affect your internet speeds.
For example, I have the Blue connector by default (SC/UPC), directly connecting it to a Blue port on the ISP router I get -19.6dBm. Putting the Blue connector in the Green Port on the XX530v i get -25.7dBm. Using a coupler and patch cord to adapt the Blue connector to a Green connector (SC/UPC → SC/APC) and connecting it to the XX530v I get -21.5dBm.

Looking forward for your reply.

I changed my broadband from excitel to RailTel. They don't bind mac. Now I am getting full speed with xx530v. 
 

Also there is no issue with using blue cable with green port. So its confirmed. Issue is with tplink firmware. 

  @Yash_Kanawjia its -24 to -25

And I will not ask excitel to change blue cable to green until tplink gives any update regarding this issue.

  @shu006 Well technically you should use the correct cable. Excitel by default gives SC/UPC (blue connector), while the router has a SC/APC port (green connector). 
I had the same case so I got a SC/UPC female coupler and a SC/UPC to SC/APC patch cord from Amazon.
It may be a part of the issue, but the throttling is purely from the firmware from my opinion. What's your XPON TX & RX power. If your RX power is greater than -28dBm you should still be fine but you should still use the correct connectors for best possible signal.

today I got a call from tplink. They are asking to replace blue fibre patch cord with green one. Will it help ? 

  @Yash_Kanawjia I contacted excitel. The denied to unbind mac. 
 

I found some old firmware but there is no way to downgrade as 0.7 is not downgradable. Let us know if you find any way to downgrade. 

  @shu006 Hey. I just received one file from TP-Link executive, I tried flashing it but because i think its older firmware its not letting me flash it. I have informed him and he'll get back to me tommrow.
If you haven't updated yours yet, try this file and let me know if it fixes the issue-
XX530v_UP_BOOT_agc3000(beta)_2025-03-21_13.52.36

I don't know the exact firmware version but this version is older than both 0.7.0 and 0.6.0