Even if it was worth near zero (market price between strikes, very close to expiry), Robinhood holds collateral for the difference. ie, if I sell a $50P and buy a $45P, Robinhood holds $500 cash collateral to cover the difference. The maximum you can lose, once thing are settled, is the difference between the strikes less the premium collected.

With a put spread, if I sell $50P and buy $45P, the holder of the $50P I sold can exercise at any point, and I have to buy his 100 shares at $50/ea. Let's say I got a $2.50 premium ($250 total) on that spread. At that point, my account is -$4750, +1 $45P. Perhaps the premium on the $45P is $0.50, so my account's nominal value is -$4700, +100 shares at this point.

However, my $45P expires in 3 days, so maybe I want to hold on and see if the shares I had to buy at $50 will increase to $55, my $45P expires worthless, and I can then sell them for $5500 total, leaving my account +$800. Or, worst case in the other direction, the price drops below $45, and I exercise my $45P to sell them for $4500, leaving my account -$250. But until expiry or assignment, my account's nominal value is in the red.

Why would it be zero? Seems like he sold a credit spread. One leg can be exercised which means it gets converted into a large equity position, which will then be offset by the other leg being exercised by the broker when they get around to it.

Since it's a defined risk trade, you can easily sell a large number of contracts which can result in very large stock positions when exercised. It's not an issue, the math works out, but it's something you need to understand before making the trade.