first blockchain buy should put my in the safe position, but this didn't happend. Why?
The word "safe" in "Alice's position is safe again" is only from the POV of the blockchain.
It is true that Alice will remain in margin call territory until either the price feed returns to its original position, or Alice updates her short position. In a liquid market this means that in fact the entire short position will be matched with orders on the market.
I have 1000 USD DEBT and 300 USD on my Balance
and 1 sec ago i was Margin Called
Your balance is not touched. The margin call is only executed against the market.