> 50% discount

There are liquidity preferences, nobody took a haircut, they may not made a lot of money as long as the sale price($5.1B) is greater than funds raised($1.2B) everyone made some money not as much as they thought, but nevertheless some.

The reason may be different than you think, Capital One is known for its aggressive marketing campaigns and physical mail spam, it is more likely they didn't want to upset the customers and end users on what Capital One will mean

It is quite likely Capitial one will mine the data, monetize the brand, sell other products and target high value users the typical Brex user.