Tried using tiger broker yesterday and tested by buying 4 shares of Palantir at 24.53 and sold all of them at 25.25, however recorded as realised profit of -1.38?

When you make a trade on US markets using Tiger Brokers, Tiger charges both commission (min 0.99 USD) and a platform fee (min 1 USD). When you use a commission-free voucher, only the commission is waived.

In my experience, they will charge both the commission and platform fee upfront, and then credit you the amount of commission paid at a later time. The cost of commissions and platform fees is probably factored into the calculation and hence your realized profit is negative.​​​