Every short order I place reduces buying power by around 1.2 times the price of the order, regardless of whether it is a market, limit, or and order that has already been filled. Could someone explain why this is? This does not happen with buy side orders.
On a related note, sometimes the margin requirement becomes 100% for certain shortable stocks. For example, recently I tried to short Palintir (PLTR), and buying power decreased by 4x the order value! Meaning I could not use any margin for that order at all (I am flagged as a day trader). PLTR trades way above the $5 limit for fully backed shorts, so I have no idea why this is.
Any help would be appreciated. Especially for the second question, because I need to know which securities cannot be bought/sold on margin in order to properly weigh my portfolio.