Its not a bug. Bank Heist is a gamble against the house, instead of against each other. So if they win they get more credits and if they lose the bot takes away credits.
The bigger the bank, the harder it is to win, but if you do win, you walk away with a bigger pot.
The multipliers are listed below.
Win Multiplier
DeepBot Federal Reserve 2.75
DeepBot National Bank 2.25
DeepBot State Bank 2
DeepBot City Bank 1.7
DeepBot Municipal Bank 1.5