Does Interactive Brokers auto convert unused USD back to SGD?

Newbie here. I checked the July statement and I can see that I bought USD on their platform, bought ETFs and 2 days later, I noticed that it converted back all the remaining USD to SGD at a different rate but 0 commission.

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I can see that my ending cash is all in SGD and 0 USD.

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Is this their SOP?

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Or how can I set whatever USD I have will stay as USD?