Last week support at last resolved my issue. It was a technical problem. I flipped a long BTCUSD long position into a short position. Original funding cost in USD was applied to the new positions to the same amount but in BTC. Bitfinex resolved it more than fairly - they closed my short 21 BTC position at the price when I tried to close it manually ($3999.1). So they are definitely not stealers, but you have to wait for ages for reply from support.