1up bike rack - bike fell off - what did we do wrong?

1up Bike Rack advice needed. Bike fell off rack on first trip. What did we do wrong?

We bought a 1Up Super Duty single tray rack to transport a Super 73-S2 SE ebike.  We loaded the bike which had a rear flat tire so it did not seat well in the tray.  We drove 3 blocks at no more than 25 miles per hour and the bike fell off when turning after a stop.

What do we need to prevent this? We have wheel locks that go through holes in the arms but did not use them. Are there straps that hold the wheel tight to the tray? If wheel locks are used and some straps to hold the tires to the tray, is that sufficient to hold the bike?

The front and rear arms were cinched very tight against the tires. It seemed solid after loading. What did we do wrong? Was it because of the rear flat tire? How can we prevent this in the future? My nephew is not happy this happened before the holiday weekend. Thank you.

This is the bike.  https://super73.com/products/super73-s2-se?_pos=1&_fid=8dd02ad58&_ss=c&from=electric-bikes