I tend to agree with you about this. I don’t think there is a made up stable coin called USDstable like the OP is mentioning.
FTX treats fiat USD as fiat USD, there is no need to differentiate. The majority of trading pairs on FTX are indeed fiat USD > crypto coin.
You’re not trading stableUSD/BTC on FTX, you’re trading USD/BTC.
I believe the FTX API is providing the correct data to Koinly, and I am sure the Koinly techs have looked at this issue and are perhaps not sure how to resolve it.
For now, whenever I deposit or withdraw a stable coin from FTX, I need to enter a manual trade transaction in Koinly.
Eg. I send 500 BUSD from FTX to Binance at 4:30pm. I now need to go and enter a manual trade transaction at 4:25pm which says “Sell USD > Buy 500 BUSD”.
There is no cost basis or CGT event because it’s 1:1.
The only time I’ve seen a CGT event is when selling stable coin I’ve been holding for a while and the foreign exchange rate has changed (USD is not my local currency).
I really hope Koinly can find a fix or workaround for this because entering these manual transactions can be tiring if there’s a lot of them.