Alleghany corp (Y)

I held a position in Y, and had it listed to sell with a trailing stop loss.

The company was recently acquired by Berkshire Hathaway.

My trailing stop loss was cancelled, and my position in Y disappeared. There was no replacement with cash or alternative stock as far as i can tell. Is this an oversight for a paper trading account? its frustrating as i had to dig to figure this out as it impacts my measure of performance of how my simulations are running.