I just messaged Coolfire02 on Discord about this but I was Prestige 21 [Free] and had acquired enough to Prestige. I auctioned it off just like I do every other time and then did /prestige. It said my balance was reset to 10,000 and I am currently P21 [A]. I don't know why my prestige didn't go up but I just lost 750 billion so if you could help me out that would be awesome. You can see in the images where I was Prestige 21 and then did /prestige and was still P21. You can also check the logs for further proof.
This is when I was P21 [Free]
This is proof that I prestiged and was still P21 as you can see when I sent a message.
I also have video proof that I did execute the /prestige command and did prestige.
Here is the video proof.